Add downsampling metrics

[mattrco]

When compact goes into its downsampling step, no app metrics are reported, which makes our dashboard look a bit bare for several hours (sometimes days if we have a backlog). Adding a couple of metrics means we can alert if no compactions or downsamples have happened in x period.

This is a little less clean than I'd like due to the two callers of downsampleBucket having slightly different contexts, however I think it's a reasonable compromise. Let me know what you think!

The metrics are labelled in the same way as compact metrics for ease of comparison.

Changes

Add thanos_compact_downsample_total and thanos_compact_downsample_failures_total downsampling metrics.

Verification

Tests are passing, will run with real workloads.

[bwplotka]

I think it makes total sense (: Thanks.

Let's move to MustRegister and then LGTM. 👍

[mattrco]

Thanks for reviewing, have updated to use MustRegister 👍

[bwplotka]

Awesome, one nit, but not a blocker (:

[bwplotka]

I think it can be private, right?

[GiedriusS]

One minor nit. Let me know if it makes sense. I think doing like I suggested makes the code base cleaner - there's less chance that we will have clashes between different "subsystems" who might try to register the same metrics

[GiedriusS]

Could we use extprom.WrapRegistererWithPrefix("thanos_compact_downsample", reg) here?

[bwplotka]

I think it makes sense but with thanos_compact prefix, but not a blocker.

[GiedriusS]

Then we would only have total here

[bwplotka]

Merging in this form, we can improve that later once we actually need to reuse this component.

[bwplotka]

Thanks @mattrco !