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.

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[rchitale7]

why would this be null? Is this just defensive?