Add instrumentation classes for projects_with_alerts_created metric

What does this MR do and why?

This MR add missing instrumentation class for projects_with_alerts_created metrics and removes the according code from usage_data

Db review

CountProjectsWithAlertsCreatedMetric#8 will generate the below queries.

Query without time range (all):

SELECT COUNT(DISTINCT(project_id)) FROM alert_management_alerts;

Query analyses: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19341/commands/63885 (internal link)

Time: 30.473 s
  - planning: 0.185 ms
  - execution: 30.473 s
    - I/O read: 23.287 s
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 349595 (~2.70 GiB) from the buffer pool
  - reads: 55899 (~436.70 MiB) from the OS file cache, including disk I/O
  - dirtied: 93 (~744.00 KiB)
  - writes: 0

Temp buffers:
  - reads: 21346 (~166.80 MiB)
  - writes: 21349 (~166.80 MiB)

Query with time range (28d):

SELECT COUNT(DISTINCT "alert_management_alerts"."id") FROM "alert_management_alerts" WHERE "alert_management_alerts"."created_at" BETWEEN '2023-05-10 09:02:08.103116' AND '2023-06-07 09:02:08.103183'

Query Analyses: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19341/commands/63889

Time: 4.754 s
  - planning: 0.243 ms
  - execution: 4.753 s
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 2246393 (~17.10 GiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

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Before	After

How to set up and validate locally

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MR acceptance checklist

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Related to #409632 (closed)