Docs: Add sidecar and compactor metrics

[mercenarysre]

Thanos components: add sidecar and compactor metrics; kindly review @douglascamata.

[douglascamata]

Can you please update the PR title to match the convention in the repository? My suggestion is docs: Add sidecar and compactor metrics.

By the way, after our conversation in Slack I thought there was going to be one PR per component. But don't worry, that's not a requirement for this case.

[mercenarysre]

Can you please update the PR title to match the convention in the repository? My suggestion is docs: Add sidecar and compactor metrics.

By the way, after our conversation in Slack I thought there was going to be one PR per component. But don't worry, that's not a requirement for this case.

I have updated the PR title to match the convention in the repository.

I will make sure to do one PR per component going forward; by waiting for one component PR to be merged, then updating my forked repo with the Thanos main repo and doing git pull locally.

[douglascamata]

@TomiwaAribisala-git you don't need to wait for one component PR to be merged to prepare the next. They are independent changes in totally different files. You can go ahead and open all the PR at the same time if you want. Up to you. 👍

[mercenarysre]

new compactor metrics added.

[mercenarysre]

Updated Sidecar and Compactor metrics, @douglascamata.