Scheduler: Leader-Slot metrics for Scheduler

[apfitzge]

Problem

While interval-reported metrics show a more complete picture, it is often the first step of debugging to check stats on a non-specific leader. This is not easily done by checking the interval-reported metrics.

Summary of Changes

Collect and report count/timing metrics for scheduler at leader-slot level.

Fixes #

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 70.88123% with 76 lines in your changes are missing coverage. Please review.

Project coverage is 81.6%. Comparing base (31a73ab) to head (242e023). Report is 1 commits behind head on master.

:exclamation: Current head 242e023 differs from pull request most recent head 74842e8. Consider uploading reports for the commit 74842e8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #35087     +/-   ##
=========================================
- Coverage    81.7%    81.6%   -0.2%     
=========================================
  Files         834      834             
  Lines      224197   224877    +680     
=========================================
+ Hits       183287   183558    +271     
- Misses      40910    41319    +409