Add count projects with monitor enabled metric

What does this MR do and why?

Adds a new metric "Count projects with Monitor enabled".

Related to #382972 (closed)

Query

SELECT COUNT("project_features"."id")
FROM "project_features"
WHERE "project_features"."monitor_access_level" != 0 AND "project_features"."id" BETWEEN 1 AND 10000

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/15015/commands/52209

Query optimization MR

Add partial index on project_features.id (!109461 - closed)

Based on the following thread now we're trying to reduce batch size from 100k to 10k and avoid adding an index.

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