[Cases] Adding file telemetry (#152968)

This PR adds telemetry for the attachment framework and files
attachments by plugin and for all cases.

Issue: #151933

Notable changes:
- We now send information for each registered attachment type
- We send average file size
  - We leverage the files saved object to aggregate this information
- We use an array type so that we don't have to hard code each
attachment type, they are dynamically discovered instead
- Stats for the attachments are done for all cases and broken down by
plugin

<details><summary>Example telemetry payload</summary>

```
                        "cases": {
                            "cases": {
                                "all": {
                                    "total": 1,
                                    "daily": 1,
                                    "weekly": 1,
                                    "monthly": 1,
                                    "status": {
                                        "open": 1,
                                        "inProgress": 0,
                                        "closed": 0
                                    },
                                    "syncAlertsOn": 1,
                                    "syncAlertsOff": 0,
                                    "totalUsers": 1,
                                    "totalParticipants": 1,
                                    "totalTags": 1,
                                    "totalWithAlerts": 0,
                                    "totalWithConnectors": 0,
                                    "latestDates": {
                                        "createdAt": "2023-03-09T15:20:46.399Z",
                                        "updatedAt": "2023-03-09T15:21:02.399Z",
                                        "closedAt": null
                                    },
                                    "assignees": {
                                        "total": 0,
                                        "totalWithZero": 1,
                                        "totalWithAtLeastOne": 0
                                    },
                                    "attachmentFramework": {
                                        "externalAttachments": [
                                            {
                                                "type": ".files",
                                                "average": 4,
                                                "maxOnACase": 4,
                                                "total": 4
                                            }
                                        ],
                                        "persistableAttachments": [],
                                        "files": {
                                            "averageSize": {
                                                "value": 3
                                            },
                                            "average": 4,
                                            "maxOnACase": 4,
                                            "total": 4
                                        }
                                    }
                                },
                                "sec": {
                                    "total": 0,
                                    "daily": 0,
                                    "weekly": 0,
                                    "monthly": 0,
                                    "attachmentFramework": {
                                        "externalAttachments": [],
                                        "persistableAttachments": [],
                                        "files": {
                                            "average": 0,
                                            "averageSize": 0,
                                            "maxOnACase": 0,
                                            "total": 0
                                        }
                                    },
                                    "assignees": {
                                        "total": 0,
                                        "totalWithZero": 0,
                                        "totalWithAtLeastOne": 0
                                    }
                                },
                                "obs": {
                                    "total": 0,
                                    "daily": 0,
                                    "weekly": 0,
                                    "monthly": 0,
                                    "attachmentFramework": {
                                        "externalAttachments": [],
                                        "persistableAttachments": [],
                                        "files": {
                                            "average": 0,
                                            "averageSize": 0,
                                            "maxOnACase": 0,
                                            "total": 0
                                        }
                                    },
                                    "assignees": {
                                        "total": 0,
                                        "totalWithZero": 0,
                                        "totalWithAtLeastOne": 0
                                    }
                                },
                                "main": {
                                    "total": 1,
                                    "daily": 1,
                                    "weekly": 1,
                                    "monthly": 1,
                                    "attachmentFramework": {
                                        "externalAttachments": [
                                            {
                                                "type": ".files",
                                                "average": 4,
                                                "maxOnACase": 4,
                                                "total": 4
                                            }
                                        ],
                                        "persistableAttachments": [],
                                        "files": {
                                            "averageSize": {
                                                "value": 3
                                            },
                                            "average": 4,
                                            "maxOnACase": 4,
                                            "total": 4
                                        }
                                    },
                                    "assignees": {
                                        "total": 0,
                                        "totalWithZero": 1,
                                        "totalWithAtLeastOne": 0
                                    }
                                }
                            },
                            "userActions": {
                                "all": {
                                    "total": 5,
                                    "daily": 5,
                                    "weekly": 5,
                                    "monthly": 5,
                                    "maxOnACase": 5
                                }
                            },
                            "comments": {
                                "all": {
                                    "total": 0,
                                    "daily": 0,
                                    "weekly": 0,
                                    "monthly": 0,
                                    "maxOnACase": 0
                                }
                            },
                            "alerts": {
                                "all": {
                                    "total": 0,
                                    "daily": 0,
                                    "weekly": 0,
                                    "monthly": 0,
                                    "maxOnACase": 0
                                }
                            },
                            "connectors": {
                                "all": {
                                    "all": {
                                        "totalAttached": 0
                                    },
                                    "itsm": {
                                        "totalAttached": 0
                                    },
                                    "sir": {
                                        "totalAttached": 0
                                    },
                                    "jira": {
                                        "totalAttached": 0
                                    },
                                    "resilient": {
                                        "totalAttached": 0
                                    },
                                    "swimlane": {
                                        "totalAttached": 0
                                    },
                                    "maxAttachedToACase": 0
                                }
                            },
                            "pushes": {
                                "all": {
                                    "total": 0,
                                    "maxOnACase": 0
                                }
                            },
                            "configuration": {
                                "all": {
                                    "closure": {
                                        "manually": 0,
                                        "automatic": 0
                                    }
                                }
                            }
                        },
```

</details>

## Testing

To test modify this file:
https://github.com/elastic/kibana/blob/main/x-pack/plugins/cases/server/telemetry/schedule_telemetry_task.ts

With:

```
export const scheduleCasesTelemetryTask = (
  taskManager: TaskManagerStartContract,
  logger: Logger
) => {
  (async () => {
    await taskManager
      .ensureScheduled({
        id: CASES_TELEMETRY_TASK_NAME,
        taskType: CASES_TELEMETRY_TASK_NAME,
        schedule: {
          interval: `${MINUTES_ON_HALF_DAY}m`,
        },
        scope: ['cases'],
        params: {},
        state: {},
      })
      .catch((err) => {
        logger.debug(
          `Error scheduling cases task with ID ${CASES_TELEMETRY_TASK_NAME} and type ${CASES_TELEMETRY_TASK_NAME}. Received ${err.message}`
        );
      });

    await taskManager.runSoon(CASES_TELEMETRY_TASK_NAME);
  })();
};
```

This will cause the telemetry to be sent as soon as the server is
restarted.

To generate files and attachments to add stats to the telemetry I
created this python script:
https://github.com/elastic/cases-files-generator

To retrieve the telemetry:

```
POST http://localhost:5601/api/telemetry/v2/clusters/_stats
{
   "refreshCache": true,
   "unencrypted": true
}
```

x-pack/plugins/cases/server/telemetry/index.ts

@@ -14,6 +14,7 @@ import type {
 import { SavedObjectsErrorHelpers } from '@kbn/core/server';
 import type { TaskManagerSetupContract } from '@kbn/task-manager-plugin/server';
 import type { UsageCollectionSetup } from '@kbn/usage-collection-plugin/server';
+import { FILE_SO_TYPE } from '@kbn/files-plugin/common';
 import { collectTelemetryData } from './collect_telemetry_data';
 import {
   CASE_TELEMETRY_SAVED_OBJECT,
@@ -42,7 +43,7 @@ export const createCasesTelemetry = async ({
 }: CreateCasesTelemetryArgs) => {
   const getInternalSavedObjectClient = async (): Promise<ISavedObjectsRepository> => {
     const [coreStart] = await core.getStartServices();
-    return coreStart.savedObjects.createInternalRepository(SAVED_OBJECT_TYPES);
+    return coreStart.savedObjects.createInternalRepository([...SAVED_OBJECT_TYPES, FILE_SO_TYPE]);
   };
 
   taskManager.registerTaskDefinitions({