[Alerting] Telemetry for calling legacy routes

[chrisronline]

Resolves #111894

This PR introduces usage counter telemetry for legacy alerting HTTP routes.

I synced with @Bamieh and he mentioned this is a good use for usage counters. I'd imagine we have some existing dashboards already charting these data points, but if not, easy to add.

@Bamieh also mentioned we should add a consistent counter_type to perform group aggregations across all data points, so I added it and it's set to legacy_api_usage for now. This isn't used anywhere else, but we could prefix alerting_ to ensure we don't conflict in the future if we want.

To test, visit any legacy alerting HTTP url and then check telemetry stats (via https://localhost:5601/api/stats?extended):

{
  "usage_counters": {
    "daily_events": [
      {
        "domain_id": "alerts",
        "counter_name": "legacy_route_list_alert_types",
        "counter_type": "count",
        "last_updated_at": "2021-09-10T16:31:07.255Z",
        "from_timestamp": "2021-09-10T00:00:00Z",
        "total": 1
      },
      {
        "domain_id": "alerts",
        "counter_name": "legacy_route_get",
        "counter_type": "count",
        "last_updated_at": "2021-09-10T16:31:27.262Z",
        "from_timestamp": "2021-09-10T00:00:00Z",
        "total": 2
      },
      {
        "domain_id": "alerts",
        "counter_name": "legacy_route_list_alert_types",
        "counter_type": "route_access",
        "last_updated_at": "2021-09-10T16:42:02.400Z",
        "from_timestamp": "2021-09-10T00:00:00Z",
        "total": 1
      },
      {
        "domain_id": "alerts",
        "counter_name": "legacy_route_get",
        "counter_type": "route_access",
        "last_updated_at": "2021-09-10T16:42:02.399Z",
        "from_timestamp": "2021-09-10T00:00:00Z",
        "total": 1
      }
    ]
  }
}

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[YulNaumenko]

LGTM!

[ymao1]

LGTM! Just some nits about the naming.

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 0a81242
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Metrics [docs]

✅ unchanged

History

• 💚 Build #152632 succeeded d6340a7
• 💚 Build #152606 succeeded 4f2abfe
• 💔 Build #152598 failed 06cc5ed

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