Fix GRPC_BYTES_SENT broker metric for gRPC query server

[yashmayya]

• This metric was introduced in Add BrokerGrpcServer for a streaming response fetching endpoint #15088 but never used.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.40%. Comparing base (e5dba19) to head (bf74385).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17422      +/-   ##
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+ Coverage     55.68%   63.40%   +7.72%     
- Complexity      703     1474     +771     
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  Files          2463     3154     +691     
  Lines        139013   187924   +48911     
  Branches      22166    28761    +6595     
============================================
+ Hits          77407   119155   +41748     
- Misses        55096    59541    +4445     
- Partials       6510     9228    +2718     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (?)
integration	100.00% <ø> (?)
integration1	100.00% <ø> (?)
integration2	0.00% <ø> (?)
java-11	63.38% <100.00%> (+7.72%)	⬆️
java-21	63.33% <100.00%> (+7.67%)	⬆️
temurin	63.40% <100.00%> (+7.72%)	⬆️
unittests	63.40% <100.00%> (+7.71%)	⬆️
unittests1	55.70% <ø> (+0.02%)	⬆️
unittests2	34.06% <100.00%> (?)

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[Copilot]

Pull request overview

This PR fixes the GRPC_BYTES_SENT broker metric that was introduced in a previous PR but never actually recorded. The fix adds proper tracking of serialized response sizes across all gRPC response paths in the broker's query server.

Key Changes

• Tracks and records the total bytes sent for each gRPC query response by accumulating serialized sizes across multiple response blocks (metadata, schema, data)
• Adds metric recording for error paths (SQL parsing errors and empty results) where responses are sent as single blocks
• Introduces comprehensive test coverage for the metric across different query scenarios (errors, empty results, and successful queries with data)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pinot-broker/src/main/java/org/apache/pinot/broker/grpc/BrokerGrpcServer.java	Adds GRPC_BYTES_SENT metric tracking by recording serialized size after each response block is sent and accumulating total bytes for multi-block responses
pinot-broker/src/test/java/org/apache/pinot/broker/grpc/BrokerGrpcServerTest.java	Adds new test class with four test methods validating the GRPC_BYTES_SENT metric is correctly recorded for SQL parsing errors, empty results, successful queries, and alongside the existing GRPC_BYTES_RECEIVED metric