Usage Ping for Prometheus and Generic Alert services

The scope of this issue is adding the following metrics to the Usage Ping:

Prometheus

• Count how many GitLab instances have Prometheus installed (Implemented in !16231 (merged))
• Count how many GitLab instances are connected to an external instance of prometheus (Implemented in !16231 (merged))

Generic Alert Endpoint

• Count how many instances have at least one project where the generic alert endpoint is enabled (Implemented in !19765 (merged))