rpc_util: Fix `RecvBufferPool` deactivation issues

[hueypark]

Fixes #6578

• Resolved unintended deactivation scenarios:

  • During unary RPC calls.
  • When compression is enabled.

• Also corrected missing payInfo.uncompressedBytes in Server.processUnaryRPC.

RELEASE NOTES:

• rpc_util: The RecvBufferPool option is now active during unary RPC and in compression is enabled.
• stats: The stats handler now accurately tracks uncompressedBytes during unary RPC calls.

[codecov[bot]]

Codecov Report

Merging #6766 (389c89e) into master (70f1a40) will decrease coverage by 6.70%. Report is 97 commits behind head on master. The diff coverage is 59.37%.

Additional details and impacted files

[hueypark]

This PR will fix https://github.com/grpc/grpc-go/issues/6578.

[zasweq]

I'm going to go ahead and approve this, I'm going to be out the next few weeks so once you get to my minor nits one of my teammates can merge this.