Re-evaluate thanos-in-kubernetes solutions

We did an issue at the beginning of this epic called "evaluate thanos operators" - https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10985.

This issue implicitly assumed that we needed an operator at all! What we arguably should have done was to evaluate thanos-in-kubernetes solutions regardless of how they are implemented.

In addition, since this epic was started, and after long pauses of working on other things, we've generally moved away from Helm to Tanka on the observability team. Jsonnet-based solutions are therefore particularly attractive.

Candidates:

• https://github.com/thanos-io/kube-thanos