Track count of alerts configured in the usage ping

We should add a new entry to the usage ping to track the usage of our alerts feature.

Metric:

• Number of projects with at least one alert created

Helpful references:

• Usage ping information: https://docs.gitlab.com/ee/user/admin_area/settings/usage_statistics.html#usage-ping
• Original MR to grab number of projects with Prometheus enabled: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1576/diffs