Optimize projects_with_prometheus_alerts query

What does this MR do?

Optimizes the projects_with_prometheus_alerts counter query for batch counting in usage data

Part of issue #208923 (closed)

Query

SELECT COUNT(DISTINCT “prometheus_alerts”.“project_id”) FROM “prometheus_alerts” WHERE “prometheus_alerts”.“project_id” BETWEEN 0 AND 9999;

Optimization

No optimization needed, see: https://explain.depesz.com/s/W6NE

Timing

Time: 0.315 ms

• planning: 0.151 ms
• execution: 0.164 ms
  • I/O read: 0.000 ms
  • I/O write: 0.000 ms

Shared buffers:

• hits: 6 (~48.00 KiB) from the buffer pool
• reads: 0 from the OS file cache, including disk I/O
• dirtied: 0
• writes: 0