Proposal: Create a new "Per LabelSet" limit

[alanprot]

What this PR does:

This pull request (PR) introduces a new limit Active Series "Per LabelSet." This limit functions similarly to -ingester.max-series-per-metric, but allowing users to define the maximum number of series per LabelSet.

These limits are configurable via runtime settings and with the following format:

max_series_per_label_set:
  - label_set:
      labelName1: LabelValue1
    limit: 10
  - label_set:
      labelName1: LabelValue1
      labelName2: LabelValue2
    limit: 3

The limits are applied "Per labelset", meaning that only metrics containing all the labels of the limit will be subjected to it.

Additionally, a new metric is implemented to monitor the active timeseries for each limit as follows:

# HELP cortex_ingester_active_series_per_labelset Number of currently active series per user and labelset.
# TYPE cortex_ingester_active_series_per_labelset gauge
cortex_ingester_active_series_per_labelset{labelset="{label1=\"value1\"}",user="1"} 3
cortex_ingester_active_series_per_labelset{labelset="{label2=\"value2\"}",user="1"} 2
cortex_ingester_active_series_per_labelset{labelset="{comp1=\"compValue1\", comp2=\"compValue2\"}",user="1"} 2
cortex_ingester_active_series_per_labelset{labelset="{comp1=\"compValue1\"}",user="1"} 7
cortex_ingester_active_series_per_labelset{labelset="{comp2=\"compValue2\"}",user="1"} 2

The motivation behind these limits is to enable service administrators to set constraints on a per labelset basis. This functionality helps mitigate the risk of unexpected increases in the number of timeseries.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[alanprot]

I still need to update the changelog but I think we can start reviewing it.

cc @yeya24 @friedrich-at-adobe

[yeya24]

Thanks

[danielblando]

Dont we want to create a new discarded samples reason label here?

[alanprot]

indeed.

I will create another PR with this and the chngelog in a bit.

[jeromeinsf]

Should https://cortexmetrics.io/docs/configuration/configuration-file/ be updated?