Make metrics stale time configurable

[spacewander]

One idea is to provide a configuration singleton and put fields like refreshMetricsInterval in it. So far, we have to pass these fields across several layers.

Originally posted by @liu-cong in #223 (comment)

[liu-cong]

Note: This issue should also deal with metrics. One idea is to put pods with stale metrics only as backups (when there are no other pods left).

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