Stats API Call timeout exception

[jmazanec15]

GET /_opendistro/_anomaly_detection/stats occasionally results in the following error message:

{
  "error": {
    "root_cause": [
      {
        "type": "timeout_exception",
        "reason": "Cannot get response within time limit: SearchRequest{searchType=QUERY_THEN_FETCH, indices=[.opendistro-anomaly-detectors], indicesOptions=IndicesOptions[ignore_unavailable=false, allow_no_indices=true, expand_wildcards_open=true, expand_wildcards_closed=false, allow_aliases_to_multiple_indices=true, forbid_closed_indices=true, ignore_aliases=false, ignore_throttled=true], types=[], routing='null', preference='null', requestCache=null, scroll=null, maxConcurrentShardRequests=0, batchedReduceSize=512, preFilterShardSize=128, allowPartialSearchResults=true, localClusterAlias=null, getOrCreateAbsoluteStartMillis=-1, ccsMinimizeRoundtrips=true, source={\"size\":0}}"
      }
    ],
    "type": "timeout_exception",
    "reason": "Cannot get response within time limit: SearchRequest{searchType=QUERY_THEN_FETCH, indices=[.opendistro-anomaly-detectors], indicesOptions=IndicesOptions[ignore_unavailable=false, allow_no_indices=true, expand_wildcards_open=true, expand_wildcards_closed=false, allow_aliases_to_multiple_indices=true, forbid_closed_indices=true, ignore_aliases=false, ignore_throttled=true], types=[], routing='null', preference='null', requestCache=null, scroll=null, maxConcurrentShardRequests=0, batchedReduceSize=512, preFilterShardSize=128, allowPartialSearchResults=true, localClusterAlias=null, getOrCreateAbsoluteStartMillis=-1, ccsMinimizeRoundtrips=true, source={\"size\":0}}"
  },
  "status": 500
}

This is a result of an expensive timed blocking call here to count up the documents in the index via Search. (1) SearchRequest should be replaced with an IndicesStatsRequest and (2) the call should be converted to an asynchronous call outside of the transport layer.