Add CPU & Memory metrics to k8s cluster page

Merged Paweł Chojnacki requested to merge 5029-support-cluster-metrics into master

What does this MR do?

Copied https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17139 with squashed changes

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #5029 (closed)

Edited by Grzegorz Bizon