[Schema] Update buckets type in metrics mappings

[kavithacm]

Which domain protocol is relevant for this schema ?

The issue is to update buckets type to nested in the observability metrics mapping to enable PPL to search and aggregate the buckets fields across documents directly. This change would help eliminate the manual process of retrieving all documents and aggregating buckets respectively in the frontend. The change would be as below.

Old mapping:

"buckets" : {
          "properties" : {
            "count" : {
              "type" : "long"
            },
            "max" : {
              "type" : "float"
            },
            "min" : {
              "type" : "float"
            }
          }
        },

New Mapping:

"buckets" : {
          "type" : "nested",
          "properties" : {
            "count" : {
              "type" : "long"
            },
            "max" : {
              "type" : "float"
            },
            "min" : {
              "type" : "float"
            }
          }
        },

What is the schema resource ?
The schema file which needs to be updated - https://github.com/opensearch-project/opensearch-catalog/blob/main/schema/observability/metrics/metrics-1.0.0.mapping#L137-L152