add prom metrics to frontend

[katherinelc321]

What this PR does

Before this PR:
Frontend has no monitoring
After this PR:
Added prometheus metrics collector and init
Jira: https://issues.redhat.com/browse/ARO-6284

Special notes for your reviewer

curling /metrics shows this:

curl -X GET 0.0.0.0:8443/metrics
# HELP frontend_count 
# TYPE frontend_count counter
frontend_count{api_version="",code="200",route="/healthz/ready",verb="GET"} 1
frontend_count{api_version="",code="200",route="/metrics",verb="GET"} 2
# HELP frontend_duration 
# TYPE frontend_duration gauge
frontend_duration{api_version="",code="200",route="/healthz/ready",verb="GET"} 0
frontend_duration{api_version="",code="200",route="/metrics",verb="GET"} 0
# HELP frontend_health 
# TYPE frontend_health gauge
frontend_health{endpoint="/healthz/ready"} 1

Checklist

This checklist is not enforcing, but it's a reminder of items that could be relevant to every PR.
Approvers are expected to review this list.

• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Deployment: The deployment process was considered and addressed if required
• Testing: New code requires new unit tests.
• Documentation: Is the documentation updated? Either in the doc located in focus area, in the README or in the code itself.
• Customers: Is this change affecting customers? Is the release plan considered?

[mbarnes]

Small change requested to eliminate a redundant constant.

[bennerv]

one comment, otherwise lgtm

[AldoFusterTurpin]

Thank you for this. 🙂

Just a couple of questions about why not to use the metrics.Emitter interface when possible in the parameters and a question about (*PrometheusEmitter).EmitGauge... syntax.

Thanks!

[mbarnes]

It looks like you addressed all of @AldoFusterTurpin's requested changes, so I'm going to dismiss his review in order to unblock this.

Requested changes have been addressed.