[Multi-stage] Reduce the stats transferred

[Jackie-Jiang]

Partially solve #12426

• Send metadata only to one mailbox as all the metadata will be aggregated at the root level
• When trace is disabled, do not collect stage stats except for the leaf stage (needed to propagate the v1 stats for compatibility)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.14286% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 61.56%. Comparing base (59551e4) to head (4cfaf1a).
Report is 29 commits behind head on master.

Files	Patch %	Lines
...requesthandler/MultiStageBrokerRequestHandler.java	0.00%	7 Missing ⚠️
...query/runtime/operator/exchange/BlockExchange.java	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12517      +/-   ##
============================================
- Coverage     61.75%   61.56%   -0.19%     
  Complexity      207      207              
============================================
  Files          2436     2450      +14     
  Lines        133233   133517     +284     
  Branches      20636    20688      +52     
============================================
- Hits          82274    82204      -70     
- Misses        44911    45229     +318     
- Partials       6048     6084      +36     

Flag	Coverage Δ
custom-integration1	<0.01% <0.00%> (-0.01%)	⬇️
integration	<0.01% <0.00%> (-0.01%)	⬇️
integration1	<0.01% <0.00%> (-0.01%)	⬇️
integration2	0.00% <0.00%> (ø)
java-11	<0.01% <0.00%> (-61.71%)	⬇️
java-21	61.56% <77.14%> (-0.06%)	⬇️
skip-bytebuffers-false	61.54% <77.14%> (-0.21%)	⬇️
skip-bytebuffers-true	61.53% <77.14%> (+33.81%)	⬆️
temurin	61.56% <77.14%> (-0.19%)	⬇️
unittests	61.56% <77.14%> (-0.19%)	⬇️
unittests1	46.69% <96.42%> (-0.20%)	⬇️
unittests2	27.67% <0.00%> (-0.06%)	⬇️

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