Emit GKE PDCSI component version metric

[saikat-royc]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
This PR enables the gce-pd-driver container to emit 'component_version' metric. This metric will be emitted only by the controller driver gce-pd-driver container, when the env GKE_PDCSI_VERSION is setup in the driver manifest.
In GKE the env variable would be populated from metadata annotations (which is automatically populated for GKE pod). This metric will enable GroupBy queries on PDCSI component version

- name: GKE_PDCSI_VERSION
      valueFrom:
        fieldRef:
          # Component version annotation is populated by renderer
          fieldPath: metadata.annotations['components.gke.io/component-version']

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:
Tested locally as follows:

1. deploy the driver (with a --http-endpoint=:22015 and env: -name:GKE_PDCSI_VERSION value: )
   sample patch:

@@ -138,9 +138,12 @@ spec:
          args:
            - "--v=5"
            - "--endpoint=unix:/csi/csi.sock"
+            - "--http-endpoint=:22015"
          env:
            - name: GOOGLE_APPLICATION_CREDENTIALS
              value: "/etc/cloud-sa/cloud-sa.json"
+            - name: GKE_PDCSI_VERSION
+              value: "0.9.4"
          volumeMounts:
            - name: socket-dir
              mountPath: /csi

2. ssh into the node where the controller is running
3. curl to the http endpoint as follows and check the metric emitted:

saikatroyc@e2e-test-saikatroyc-minion-group-s7lb ~ $ curl http://localhost:22015/metrics
# HELP component_version [ALPHA] Metric to expose the version of the PDCSI GKE component.
# TYPE component_version gauge
component_version{component_version="0.9.4"} 1

Does this PR introduce a user-facing change?:

Emit GKE component version metric 

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: saikat-royc

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[saikat-royc]

/assign @verult @mattcary

[saikat-royc]

/retest

[Jiawei0227]

Can I ask what is benefit of adding this version through metrics? I think we have other ways to get the component version number through GKE

[Jiawei0227]

I remember in GKE we have add_on_* something and that will contain the component versions.

[mattcary]

I remember in GKE we have add_on_* something and that will contain the component versions.

GKE is moving to a different component release pipeline which changes how versions are exported.

[verult]

Just a note that a mux is only necessary if there are multiple handlers, so here either this or the previous version of this code would work

[saikat-royc]

Keeping mux for future proofing?

[saikat-royc]

/retest

[Jiawei0227]

/lgtm