CBG-4312 improve metrics documentation

[torcolvin]

CBG-4312

• remove /_metrics endpoint, which does not need public API documentation, as it never should have been implemented
• change copy to match public docs for description
• add links to upstream files

This upstreams changes done in https://github.com/couchbase/docs-sync-gateway/pull/839/files

@simon-dew, I did not make the same changes in the files https://github.com/couchbase/sync_gateway/blob/main/docs/api/metric-capella.yaml and https://github.com/couchbase/sync_gateway/blob/main/docs/api/admin-capella.yaml . I am happy to make the corresponding changes in these files or we can wait until you figure out what the appropriate externalDocs sections are.

Can you verify that I ported all the changes? I did not port the line /_metrics because I think it is wrong, we can wait until https://github.com/couchbase/docs-sync-gateway/pull/839/files is resolved.

          application/json:
            schema:
              $ref: metrics_metadata.json

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

[github-actions]

Redocly previews

• Admin API
• Public API
• Metric API
• Diagnostic API

[simon-dew]

Looks like you have incorporated almost all of the changes from couchbase/docs-sync-gateway#839 👍

I have a couple of comments though: