[core.metrics] Add support for multiple processes in ops metrics & stats API; deprecate process field

[Bamieh]

WIP. Remaining items until ready:

• Deprecate process field
• Add name field for each process. So far we only have main_thread.
• Move event loop delay histogram to core
• Telemetry collectors to use core's histogram
• Core.metrics use histogram.mean for event_loop_delay.
• Core.metrics expose histogram to be used for stats and status api.
• Update unit tests
• Add integration tests

Summary

/api/status

{
  "processes": 
     [
	    {
	      "name": "main_thread",
	      "memory": {
	        "heap": {
	          "total_in_bytes": 387874816,
	          "used_in_bytes": 332095280,
	          "size_limit": 12935233536
	        },
	        "resident_set_size_in_bytes": 540487680
	      },
	      "pid": 29897,
	      "event_loop_delay": 11417136.036613273,
	      "event_loop_delay_histogram": {
	        "min": 9469952,
	        "max": 19021823,
	        "mean": 11417136.036613273,
	        "exceeds": 0,
	        "stddev": 1132985.4515301601,
	        "fromTimestamp": "2021-08-09T12:24:51.190Z",
	        "lastUpdatedAt": "2021-08-09T12:24:56.185Z",
	        "percentiles": {
	          "50": 11247615,
	          "75": 12599295,
	          "95": 12656639,
	          "99": 14434303
	        }
	      },
	      "uptime_in_millis": 101736.272458
	    }
	  ],
  "process": {
      "name": "main_thread",
      "memory": {
        "heap": {
          "total_in_bytes": 387874816,
          "used_in_bytes": 332095280,
          "size_limit": 12935233536
        },
        "resident_set_size_in_bytes": 540487680
      },
      "pid": 29897,
      "event_loop_delay": 11417136.036613273,
      "event_loop_delay_histogram": {
        "min": 9469952,
        "max": 19021823,
        "mean": 11417136.036613273,
        "exceeds": 0,
        "stddev": 1132985.4515301601,
        "fromTimestamp": "2021-08-09T12:24:51.190Z",
        "lastUpdatedAt": "2021-08-09T12:24:56.185Z",
        "percentiles": {
          "50": 11247615,
          "75": 12599295,
          "95": 12656639,
          "99": 14434303
        }
      },
      "uptime_in_millis": 101736.272458
    }
}

Closes #104031

[joshdover]

General direction here looking good!

[joshdover]

✅

[kibanamachine]

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Metrics [docs]

Unknown metric groups

API count

id	before	after	diff
core	2246	2263	+17

API count missing comments

id	before	after	diff
core	997	1011	+14

History

• 💔 Build #147851 failed d9faaaf
• 💔 Build #147670 failed b286f8b
• 💔 Build #145155 failed 903dabf
• 💔 Build #143705 failed 9425bf2

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@elasticmachine merge upstream

[Bamieh]

Closing in favor of #109820