discussion: histogram metrics for each vnode and each epoch

[st1page]

In #12381 we support the fragment-level metrics and hide the actor-level metrics. But actually, the actor-level metrics are useless except for finding if there is skew. And the actor-level metrics introduce pressure on the prometheus.
So to just check if there is skew, a histogram metrics and the p50, p90, p99 number based on it are enough. We can count the statics of each vnode for each epoch, and update the number of the vnode in the epoch.

[github-actions]

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