Add a metric to track samples dropped in distributor by metric_relabel

[bboreham]

If you have a metric_relabel_configs ... action: drop then those samples disappear from cortex_distributor_received_samples_total.

I'd like them to be counted in cortex_discarded_samples_total.

Relating to the feature added in #3329

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[bboreham]

Still relevant

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[bboreham]

Still relevant

[pedro-stanaka]

I will try to resolve this one this week. Trying to get involved with the project here.

[pedro-stanaka]

@bboreham I tried to solve this issue on #4503, would really appreciate the feedback there.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pedro-stanaka]

Waiting for feedback on PR, should be able to merge soon.