[Meta] Path to GA for the GitLab ruby Prometheus metrics

This issue is intended to serve as a checklist for going GA with our GitLab ruby Prometheus metrics:

• Lock down histogram buckets
• Attain influxdb metric parity, to the extent that it makes sense (not all prior metrics may be relevant, usable)
• Confirm gem is running stable in production
• Deprecate InfluxDB
• Turn on ruby metrics by default (https://gitlab.com/gitlab-org/gitlab-ce/issues/43660)
• Update documentation accordingly
• Update blog post