Replace metrics local mocks with API response

What does this MR do and why?

• Remove mocks from metrics client and hook it up to API response.
• Fix metrics label colour based on updated schema

Part of Metrics to Experiment (gitlab-org/opstrace&83)

Close gitlab-org/opstrace/opstrace#2469 (closed)

Screenshots or screen recordings

How to set up and validate locally

Unfortunately running the whole stack is not trivial. I can prepare a virtual-machine and share VPN credentials if someone wants to test it.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.