Add field type cache data to health stats API

[dzane17]

Description

Add field type cache stats to the Query Insights health_stats API.

This enables users to track total hit count, miss count, entry count, and cache memory size.

% curl -XGET "http://localhost:9200/_insights/health_stats?pretty"                     
{
  "WcXrhPnVRdmjADJAFf98aw" : {
    "ThreadPoolInfo" : {
      "query_insights_executor" : {
        "type" : "scaling",
        "core" : 1,
        "max" : 5,
        "keep_alive" : "5m",
        "queue_size" : -1
      }
    },
    "QueryRecordsQueueSize" : 0,
    "TopQueriesHealthStats" : {
      "cpu" : {
        "TopQueriesHeapSize" : 0,
        "QueryGroupCount_Total" : 0,
        "QueryGroupCount_MaxHeap" : 0
      },
      "memory" : {
        "TopQueriesHeapSize" : 0,
        "QueryGroupCount_Total" : 0,
        "QueryGroupCount_MaxHeap" : 0
      },
      "latency" : {
        "TopQueriesHeapSize" : 2,
        "QueryGroupCount_Total" : 2,
        "QueryGroupCount_MaxHeap" : 0
      }
    },
    "FieldTypeCacheStats" : {
      "size_in_bytes" : 336,
      "entry_count" : 3,
      "evictions" : 1,
      "hit_count" : 5,
      "miss_count" : 4
    }
  }
}

Issues Resolved

Resolves #146

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