Expose new /metrics endpoint to allow monitoring of addon-resizer

[raywainman]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds Prometheus metrics to addon-resizer, allowing monitoring of overall health. Initial implementation adds a single metric called addon_resizer_execution_outcome which is a simple counter metric that tracks the outcome of every addon-resizer loop (for example, whether addon-resizer actually updated the deployment for metrics-server).

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Follow up to #6764 where addon-resizer is made compatible to run in the control plane.

Does this PR introduce a user-facing change?

-Expose new `/metrics` endpoint in addon-resizer to expose health metrics that are useful for monitoring purposes.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

To use:

• Ensure the expose-metrics flag is set to true.
• The /metrics endpoint is available by default on port :8945 (configurable by the address flag).

[raywainman]

/assign chelseychen

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/assign chelseychen

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[chelseychen]

LGTM, thanks for the change!

[kwiesmueller]

/lgtm
/hold
In case you want to respond to the comments. I don't think they are blocking (even though a test would be nice, but addon-manager doesn't have too many). Feel free to remove the hold if ack.

[kwiesmueller]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kwiesmueller, raywainman

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Needs approval from an approver in each of these files:

• addon-resizer/OWNERS [kwiesmueller]

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[raywainman]

/remove-hold

Resolved comments and Kevin re-LGMT'ed and approved.