[cinder-csi-plugin] Helm Chart: add metrics and Prometheus Operator support

[desaintmartin]

What this PR does / why we need it:

add metrics and Prometheus Operator support

Which issue this PR fixes(if applicable):
fixes #2350

Special notes for reviewers:

• I separated http-server and podmonitor since the former can be used without the latter, although it requires two values to properly enable.
• I "standardised" the way we start an http-server, and it may be incompatible with a Helm Release with extraArgs values already stating this flag.

Release note:

[cinder-csi-plugin] Helm Chart: add metrics and Prometheus Operator support

[k8s-ci-robot]

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[dulek]

/approve

[k8s-ci-robot]

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This pull-request has been approved by: dulek

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[jichenjc]

/lgtm

[MPV]

Only the deployment, not the daemonset also? 🤔 😊

For comparison, here's how it was added in kops (on both):

• https://github.com/kubernetes/kops/pull/15341/files