Move over to Labkit Healthcheck endpoint

Requires labkit!35 (merged)

Currently Gitaly uses it's own custom healthcheck endpoint.

This changes moves over to the standard monitoring endpoint in LabKit.

This provides similar functionality to what Gitaly already has:

1. Prometheus/OpenMetrics endpoint
2. A PProf endpoint
3. A standardised way to configure the gitlab_build_info metric

However, it is done in a standard way. When new features are added, the will be consistent across all services.