Automatic merge from submit-queue (batch tested with PRs 59159, 60318, 60079, 59371, 57415). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Introduce External Metrics API
**What this PR does / why we need it**:
Introduce External Metrics API
This PR introduces External Metrics API which has been proposed: xref https://github.com/kubernetes/community/pull/1802
**Release note**:
```release-note
NONE
```
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