Reconsider Stable Metrics Approach via CLI flag

[mrueg]

What would you like to be added:

A way to allow stable metrics to change without increasing major version of KSM.

Why is this needed:

Stable metrics are an impediment to create alignment between all metrics and stop us from making progress in unifying the metric naming, labels etc.
It's understandable that due to the fact, that Prometheus ecosystem has a poorly defined metrics lifecycle, customers want some stability.

Describe the solution you'd like

We should have a CLI flag --stable-metrics=v2 which can be used to enforce stability if the user wants that. This can be used to create stable metrics in the individual metric collectors, while allowing us to move on with better names and better metrics at the same time.
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Additional context

[dashpole]

/assign @dgrisonnet
/triage accepted