allow gauge stored in metric registry to be updated

[zhtaoxiang]

Currently, if a gauge is created and save into a metric registry, it cannot be updated. This PR makes it changable.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 70.39%. Comparing base (b3d1ac3) to head (55adf9a).
Report is 2540 commits behind head on master.

Files with missing lines	Patch %	Lines
...not/plugin/metrics/yammer/YammerSettableGauge.java	27.27%	8 Missing ⚠️
...pache/pinot/plugin/metrics/yammer/YammerGauge.java	44.44%	5 Missing ⚠️
.../java/org/apache/pinot/spi/metrics/PinotGauge.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #9961      +/-   ##
============================================
+ Coverage     70.37%   70.39%   +0.02%     
- Complexity     5071     5657     +586     
============================================
  Files          1987     1991       +4     
  Lines        106761   107206     +445     
  Branches      16203    16289      +86     
============================================
+ Hits          75134    75472     +338     
- Misses        26385    26459      +74     
- Partials       5242     5275      +33     

Flag	Coverage Δ
integration1	25.03% <33.33%> (-0.13%)	⬇️
integration2	24.53% <33.33%> (+0.03%)	⬆️
unittests1	67.88% <77.77%> (+0.05%)	⬆️
unittests2	13.59% <0.00%> (-2.22%)	⬇️

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[snleee]

LGTM. Thank you for adding this!