Add metrics for repository interactions to RemoteIndexBuildStrategy

[jed326]

Description

This PR introduces some new metrics within the k-NN plugin related to remote index builds
New metrics:

      "remote_vector_index_build_stats" : {
        "repository_stats" : {
          "write_time_in_millis" : 15,
          "read_success_count" : 0,
          "read_failure_count" : 0,
          "write_success_count" : 1,
          "write_failure_count" : 0,
          "read_time_in_millis" : 0
        },
        "client_stats" : {
          "status_request_failure_count" : 0,
          "status_request_success_count" : 0,
          "index_build_success_count" : 0,
          "index_build_failure_count" : 0,
          "build_request_failure_count" : 1,
          "build_request_success_count" : 0,
          "waiting_time_in_ms" : 0
        },
        "build_stats" : {
          "remote_index_build_time_in_millis" : 23,
          "remote_index_build_current_size" : 0,
          "remote_index_build_current_operations" : 0
        }
      },

Complementary to these, existing repository implementations may also collect their own metrics. For example, repository-s3 collects metrics like so:
Existing repo metrics:

     "repositories" : [
        {
          "repository_name" : "vector-repo",
          "repository_type" : "s3",
          "repository_location" : {
            "bucket" : "gpu-acceleration",
            "base_path" : "vectors"
          },
          "request_time_in_millis" : {
            "ListObjects" : 49,
            "PutMultipartObject" : 0,
            "PutObject" : 199,
            "GetObject" : 38,
            "DeleteObjects" : 51
          },
          "request_retry_count_total" : {
            "ListObjects" : 0,
            "PutMultipartObject" : 0,
            "PutObject" : 0,
            "GetObject" : 0,
            "DeleteObjects" : 0
          },
          "request_failures_total" : {
            "ListObjects" : 0,
            "PutMultipartObject" : 0,
            "PutObject" : 0,
            "GetObject" : 0,
            "DeleteObjects" : 0
          },
          "generic_stats" : {
            "LowPriorityPermitsUtilization" : 0,
            "NormalPriorityPermitsUtilization" : 0,
            "LowPriorityQUtilization" : 0,
            "NormalPriorityQUtilization" : 0
          },
          "request_success_total" : {
            "ListObjects" : 1,
            "PutMultipartObject" : 0,
            "PutObject" : 6,
            "GetObject" : 3,
            "DeleteObjects" : 1
          }
        }
      ],

Related Issues

Relates #2391

Check List

• New functionality includes testing.
  - [ ] New functionality has been documented.
  - [ ] API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
  - [ ] Public documentation issue/PR created.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[rchitale7]

why would this be null? Is this just defensive?

[jed326]

Closing in favor of #2615