Automatic merge from submit-queue (batch tested with PRs 59674, 60059, 60220, 58916, 60336). 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>.

Client for external metrics

Add client for External Metrics API introduced in https://github.com/kubernetes/kubernetes/pull/60079.

This PR is just client all the API changes are included from https://github.com/kubernetes/kubernetes/pull/60079.