[Metrics UI] Add endpoint for Metrics API

[simianhacker]

Summary

This PR adds the endpoint for the Metrics API under POST /api/infra/metrics_api. This also changes the Metrics API so it will calculate the interval based on metricset.period if the user sets timerange.interval to modules and adds modules as an array of strings listing the modules.

Example

POST /api/infra/metrics_api
{
    "timerange": {
        "field": "@timestamp",
        "from": {{from}},
        "to": {{to}},
        "interval": "modules"
    },
   "modules": ["system.cpu"],
    "filter": [
        {
            "term": {
                "host.name": "2019-Macbook-Pro.local"
            }
        }
    ],
    "indexPattern": "metricbeat-*",
    "limit": 9,
    "metrics": [
        {
            "id": "user",
            "aggregations": {
                "user": {
                    "avg": {
                        "field": "system.cpu.user.pct"
                    }
                }
            }
        }
    ]
}

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[phillipb]

LGTM

[phillipb]

@elasticmachine merge upstream

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: a0030f4

Metrics [docs]

async chunks size

id	before	after	diff
infra	2.6MB	2.6MB	+196.0B

distributable file count

id	before	after	diff
default	48117	48119	+2

History

• 💛 Build #84166 was flaky bfc9942
• 💔 Build #84160 failed ea9a7b1
• 💚 Build #83826 succeeded fd98092

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@elasticmachine merge upstream