Schema change for /endpoint_metrics response

[letmaik]

(Requested by @msftsettiy)

The /endpoint_metrics endpoint currently has a response schema that is inspired by the structure of OpenAPI documents using two nested dictionaries for URL paths and methods:

{
  "metrics": 
  {
    "/foo":
    {
      "GET":
      {
        "calls": 123,
        "errors": 1,
        "failures": 5
      }
    }
  }
}

The following schema is more consistent with the rest of the API, makes schema evolution easier, and makes code of consuming clients more readable (e.g. path and method are now named concepts) and easier to write (self-contained objects with full context, path and method):

{
  "metrics": 
  [
      {
        "path": "/foo",
        "method": "GET",
        "calls": 123,
        "errors": 1,
        "failures": 5
      }
  ]
}

The only downside is that it is not immediately clear anymore that "path" and "method" are the keys.