fix: add missing protocols field to plugins

[GGabriele]

Summary

All plugin entities are expected to have a protocols schema field, but some plugin schemas are missing it. From a functional point of view, this doesn't have any impact, since the protocols field is still injected in the overall schema. But when plugin schemas are retrieved via the Admin API using the /schemas/plugins/<name> endpoint, then the "original" schema is returned. For example:

$ http :8001/schemas/plugins/prometheus
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 354
Content-Type: application/json; charset=utf-8
Date: Tue, 11 Oct 2022 06:42:47 GMT
Server: kong/3.0.0
X-Kong-Admin-Latency: 6

{
    "fields": [
        {
            "config": {
                "fields": [
                    {
                        "per_consumer": {
                            "default": false,
                            "type": "boolean"
                        }
                    },
                    {
                        "status_code_metrics": {
                            "default": false,
                            "type": "boolean"
                        }
                    },
                    {
                        "latency_metrics": {
                            "default": false,
                            "type": "boolean"
                        }
                    },
                    {
                        "bandwidth_metrics": {
                            "default": false,
                            "type": "boolean"
                        }
                    },
                    {
                        "upstream_health_metrics": {
                            "default": false,
                            "type": "boolean"
                        }
                    }
                ],
                "required": true,
                "type": "record"
            }
        }
    ]
}

This is problematic for decK, which uses this endpoint to inject schema defaults before doing a deployment, which can cause unnecessary/misleading diffs:

$ cat kong.yaml
_format_version: "3.0"
plugins:
  - name: prometheus

$ deck sync
updating plugin prometheus (global)  {
   "config": {
     "bandwidth_metrics": false,
     "latency_metrics": false,
     "per_consumer": false,
     "status_code_metrics": false,
     "upstream_health_metrics": false
   },
   "enabled": true,
   "id": "93aece4b-b813-4d9b-af28-f4c1c874f128",
   "name": "prometheus",
-  "protocols": [
-    "grpc",
-    "grpcs",
-    "http",
-    "https"
-  ]
 }

Summary:
  Created: 0
  Updated: 1
  Deleted: 0

Full changelog

• add missing protocols field to plugin schemas

[CLAassistant]

All committers have signed the CLA.

[hbagdi]

@bungle @gszr Is there a way to add an assertion in our test suite to ensure that something like this doesn't happen again (at least for bundled plugins)?

[hbagdi]

@flrgh With your new powers, can you help us review this PR?
@GGabriele Please work with @flrgh to get this over the finish line.

[flrgh]

@GGabriele code changes LGTM, thanks 👍 can you please add a CHANGELOG entry for this? We'll be good to merge once that's done.

[GGabriele]

@GGabriele code changes LGTM, thanks 👍 can you please add a CHANGELOG entry for this? We'll be good to merge once that's done.

Thanks for the review!
I just pushed the Changelog update too.