Add default partition when no matching labelset liimits

[yeya24]

What this PR does:

We added per labelset limits in Per LabelSet, which allows users to define subtenants within a user by defining labelsets and we can apply limits such as active series to throttle the usage of each subtenant.

There is a scenario where team label is used to define subtenants for a user. However, there might be a lot of teams and it is not easy to set limits for every team explicitly. Instead, users can define labelsets limits for some of the large teams and use put rest of teams using a catch all limit. This is called the default partition, which means it matches series that doesn't match any of the existing labelset limits.

Default partition doesn't have any label defined. In the example below, the second limit is the default partition and it should match all series that with name != prometheus_build_info.

    limits_per_label_set:
      - limits:
          max_series: 10000
        label_set:
          __name__: prometheus_build_info
      - limits:
          max_series: 10000
        label_set:

Metrics for default partition looks like below.

cortex_ingester_limits_per_labelset{labelset="{}",limit="max_series",user="1"} 2
cortex_ingester_usage_per_labelset{labelset="{}",limit="max_series",user="1"} 2

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]

Should we document in the label set config that a empty labelset is the "default" and will match all other series?

[yeya24]

Updated in the latest commit