Count number of projects with active Prometheus Service

What does this MR do?

Implements #263521 (closed)

Adds projects_with_prometheus_service_enabled usage ping. To count the number of projects with Prometheus Service activated.

✅ Event added to Event Dictionary

DB Queries

Generated by

::Gitlab::UsageData.count(::PrometheusService.active)

SELECT MIN("services"."id")
FROM "services"
WHERE "services"."type" = 'PrometheusService' AND "services"."active" = TRUE

• #database-lab query: https://gitlab.slack.com/archives/CLJMDRD8C/p1604936243258600
• Explain: https://explain.depesz.com/s/hp2CX

SELECT MAX("services"."id")
FROM "services"
WHERE "services"."type" = 'PrometheusService' AND "services"."active" = TRUE

• #database-lab query: https://gitlab.slack.com/archives/CLJMDRD8C/p1604936341261000
• Explain: https://explain.depesz.com/s/hTir

SELECT COUNT("services"."id")
FROM "services"
WHERE "services"."type" = 'PrometheusService' AND 
"services"."active" = TRUE AND
"services"."project_id" IS NOT NULL AND
"services"."id" >= 12 AND
"services"."id" < 100000

• #database-lab query: https://gitlab.slack.com/archives/CLJMDRD8C/p1605007020398500
• Explain https://explain.depesz.com/s/lFae
• Performs better than the query without "project_id" IS NOT NULL because it uses index_services_on_type_id_when_active_and_project_id_not_null index. For comparison:
  • #database-lab query: https://gitlab.slack.com/archives/CLJMDRD8C/p1604936417263300
  • Explain https://explain.depesz.com/s/b8DK

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team