Report number of services per namespace and type

[xvello]

What does this PR do?

Add a kubernetes_state.service.count gauge, tagged by namespace, type and service name, to monitor the number of services in a cluster.

Instead of sending one gauge per service, the same strategy as persistentvolumes.by_phase is used: count by relevant tags. The PV code has been refactored in a generic count_objects_by_tags method to do that.

Other object types could benefit from this generic client-side counting logic, to reduce the number of gauges the check sends

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

Anything else we should know when reviewing?

[codecov-io]

Codecov Report

Merging #2247 into master will decrease coverage by 3.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2247      +/-   ##
==========================================
- Coverage   85.83%   82.71%   -3.13%     
==========================================
  Files         189        4     -185     
  Lines       12646      376   -12270     
  Branches     1325       72    -1253     
==========================================
- Hits        10855      311   -10544     
+ Misses       1408       44    -1364     
+ Partials      383       21     -362

[gzussa]

LGTM

[mfpierre]

Nice use of metric transformers 👍