Add broker gauge for estimated MSE query server threads

[yashmayya]

• Adds some observability for MSE's query throttling functionality (Add cluster configuration to allow limiting the number of multi-stage queries running concurrently #14574, Rework MSE query throttling to take into account estimated number of threads used by a query #14847).
• Note that this gauge metric can be used independently of the query throttling functionality - i.e., it doesn't require pinot.beta.multistage.engine.max.server.query.threads to be configured. It provides a per broker view of the multi-stage engine query workload.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Project coverage is 63.54%. Comparing base (59551e4) to head (9af7e3d).
Report is 1829 commits behind head on master.

Files with missing lines	Patch %	Lines
...requesthandler/MultiStageBrokerRequestHandler.java	0.00%	3 Missing ⚠️
...roker/requesthandler/MultiStageQueryThrottler.java	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #15189      +/-   ##
============================================
+ Coverage     61.75%   63.54%   +1.79%     
- Complexity      207     1461    +1254     
============================================
  Files          2436     2764     +328     
  Lines        133233   155686   +22453     
  Branches      20636    23937    +3301     
============================================
+ Hits          82274    98929   +16655     
- Misses        44911    49298    +4387     
- Partials       6048     7459    +1411     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (+99.99%)	⬆️
integration	100.00% <ø> (+99.99%)	⬆️
integration1	100.00% <ø> (+99.99%)	⬆️
integration2	0.00% <ø> (ø)
java-11	63.52% <69.23%> (+1.81%)	⬆️
java-21	63.43% <69.23%> (+1.81%)	⬆️
skip-bytebuffers-false	63.53% <69.23%> (+1.79%)	⬆️
skip-bytebuffers-true	63.42% <69.23%> (+35.69%)	⬆️
temurin	63.54% <69.23%> (+1.79%)	⬆️
unittests	63.54% <69.23%> (+1.79%)	⬆️
unittests1	56.15% <100.00%> (+9.26%)	⬆️
unittests2	33.97% <69.23%> (+6.24%)	⬆️

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