cmd/helm-operator/main.go: add --metrics-addr flag

[joelanford]

Description of the change:
Add --metrics-addr flag for Helm operator

Motivation for the change:
#2451 #3358

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[camilamacedo86]

Just a nit in the fragment. Otherwise, it shows great
/lgtm

[camilamacedo86]

WDYT about we improve it and describe where the --metrics-addr=:8383 need to be used (config/default, and config/manager/)?

[joelanford]

I disagree for a few reasons:

1. In general, it should be obvious to most operator authors, and kubernetes users how to set flags on their operator.
2. In fairness, it is somewhat confusing when kustomize enters the picture, so I would propose that we write a general section in a doc somewhere about how to set flags on the manager when using the kustomize templates. This should probably be a kubebuidler doc.
3. There are lots of flags on operators. We shouldn't have to repeat ourselves in migration guides, docs, etc. anytime we discuss setting flags.

[camilamacedo86]

Far enough.

[openshift-ci-robot]

New changes are detected. LGTM label has been removed.

[joelanford]

This removal is no directly related to this PR, but needs to happen to avoid problems running the operator in a cluster.

This is an accidental carryover from Go operators that have a manager binary. But helm-operator has a different binary entrypoint: the base image adds the binary and sets the entrypoint for us, so we just need to specify the flags to pass to the helm-operator binary.