Capture metrics for ES scroll requests

[kevinconaway]

What does this PR do?

Adds metrics for ES scroll requests that are exposed in indices.search:

    "indices": {
     ...
      "search": {
        "open_contexts": 4,
        "query_total": 795996720,
        "query_time_in_millis": 23224962799,
        "query_current": 7,
        "fetch_total": 89852997,
        "fetch_time_in_millis": 128959355,
        "fetch_current": 2,
        "scroll_total": 393249911,
        "scroll_time_in_millis": 40220899351103,
        "scroll_current": 1123,
        "suggest_total": 0,
        "suggest_time_in_millis": 0,
        "suggest_current": 0
      },
      ...

cc @DataDog/burrito

Motivation

To help differentiate between scroll requests and normal queries

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

[codecov-io]

Codecov Report

Merging #2687 into master will increase coverage by 4.94%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2687      +/-   ##
==========================================
+ Coverage   83.92%   88.87%   +4.94%     
==========================================
  Files         649       11     -638     
  Lines       36471      602   -35869     
  Branches     4427       84    -4343     
==========================================
- Hits        30609      535   -30074     
+ Misses       4573       45    -4528     
+ Partials     1289       22    -1267