Add projects_with_enabled_alert_integrations usage ping

What does this MR do?

Contributes to #263521 (closed)

Add projects_with_enabled_alert_integrations usage ping to count projects with at least 1 enabled integration.

DB query

The query generated by the call:

::Gitlab::UsageData.distinct_count(::AlertManagement::HttpIntegration.active, :project_id)

Queries:

SELECT MIN("alert_management_http_integrations"."project_id")
FROM "alert_management_http_integrations"
WHERE "alert_management_http_integrations"."active" = TRUE

• #database-lab query: https://gitlab.slack.com/archives/CLJMDRD8C/p1604336682059200
• explain: https://explain.depesz.com/s/pe6J

---

SELECT MAX("alert_management_http_integrations"."project_id")
FROM "alert_management_http_integrations"
WHERE "alert_management_http_integrations"."active" = TRUE

• #database-lab query: https://gitlab.slack.com/archives/CLJMDRD8C/p1604336854061700
• explain: https://explain.depesz.com/s/k1Xl

---

SELECT COUNT(DISTINCT "alert_management_http_integrations"."project_id")
FROM "alert_management_http_integrations"
WHERE "alert_management_http_integrations"."active" = TRUE
  AND "alert_management_http_integrations"."project_id" >= 1
  AND "alert_management_http_integrations"."project_id" < 10000

• #database-lab query: https://gitlab.slack.com/archives/CLJMDRD8C/p1604337043067300
• explain: https://explain.depesz.com/s/yTsY

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