Added/tested get time EWMA to non-heap tiers for node stats response

[peteralfonsi]

Description

Adds an EWMA for get time to the node stats response for non-heap tiers, like the upcoming disk tier. Intended as a tweak for #8.

Calling _nodes/stats/indices/request_cache now returns the following:

  "_nodes": {
    "total": 1,
    "successful": 1,
    "failed": 0
  },
  "cluster_name": "runTask",
  "nodes": {
    "ze65wfe0SiO1-BiQMvCNTQ": {
      "timestamp": 1699297297377,
      "name": "runTask-0",
      "transport_address": "127.0.0.1:9300",
      "host": "127.0.0.1",
      "ip": "127.0.0.1:9300",
      "roles": [
        "cluster_manager",
        "data",
        "ingest",
        "remote_cluster_client"
      ],
      "attributes": {
        "testattr": "test",
        "shard_indexing_pressure_enabled": "true"
      },
      "indices": {
        "request_cache": {
          "memory_size_in_bytes": 1368,
          "evictions": 0,
          "hit_count": 40,
          "miss_count": 40,
          "entries": 2,
          "tiers": {
            "disk": {
              "memory_size_in_bytes": 0,
              "evictions": 0,
              "hit_count": 0,
              "miss_count": 40,
              "entries": 0,
              "get_time_ewma_millis": 0.040995631587434765
            }
          }
        }
      }
    }
  }
}```

Tested with unit tests, IT test, and manually (producing above response). 

### Related Issues
Part of larger [tiered caching](https://github.com/opensearch-project/OpenSearch/issues/10024) feature. 

### Check List
- [x] New functionality includes testing.
  - [x] All tests pass
- [x] New functionality has been documented.
  - [N/A] New functionality has javadoc added
- [x] Commits are signed per the DCO using --signoff
- [N/A] Commit changes are listed out in CHANGELOG.md file (See: [Changelog](../blob/main/CONTRIBUTING.md#changelog))
- [N/A] Public documentation issue/PR [created](https://github.com/opensearch-project/documentation-website/issues/new/choose)

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](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).