Add metrics to monitor PVCs stuck in resize

[RadaBDimitrova]

How to categorize this PR?

/area monitoring
/kind enhancement

What this PR does / why we need it:
Adds a per-PVC Gauge that exposes when an in-flight resize started, giving operators visibility into stuck or long-running resizes. This way you can set up alerts to signal if a resize gets stuck for a long time, by using the new resize_started_timestamp_seconds Gauge labelled by namespace and persistentvolumeclaim) that reports the unix timestamp at which the current in-progress resize started. It is unset for PVCs that are not mid-resize.

The PVC autoscaler sets the gauge whenever a resize is observed in progress - both for PVCs being actively scaled and for PVCs at max capacity whose resize is still in flight by using the recommendation's LastResizeTime and falling back to now when unset. It resets the Gauge at the start of each reconcile sweep (and when there are no PVCAs to reconcile), so series are cleared once a resize completes and no longer report the PVC.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
This has been developed on top of #308 in a common effort to improve metrics for the pvc-autoscaler. The first commit will be dropped once #308 gets merged.
/cc @Kostov6 @plkokanov

Release note:

Add a new `resize_started_timestamp_seconds` Gauge reporting, per PVC currently being resized, the unix timestamp at which the in-flight resize started.

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