fix(nb): reset running notebooks metrics before scraping statefulsets

[weekenthralling]

It might be better to clear the historical records each time the notebook_running metric is calculated.

For example, if I create a notebook in the test namespace and then delete it, never using the test namespace again, the notebook_running metric will still contain the record notebook_running{namespace="test"} 1.

This is because the stsCache will no longer track data related to the test namespace, and m.runningKernels.WithLabelValues(ns).Set(v) will never update the data for the test namespace.

[google-oss-prow]

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[andyatmiami]

/area v1