grpc: Fix encoded message size reported in error message

[arjan-bal]

This PR fixes two bugs in the encoded message size error message:

1. Using len(b) returns the number of buffers in a BufferSlice and not the number of bytes in the BufferSlice.
2. Finding the size after calling b.Free() would return 0 if there are no remaining references to the buffers within the slice.

RELEASE NOTES:

• grpc: Fix the number of bytes reported in the error message when encoded messages are larger than 4GB.

[dfawley]

Is there an open issue that this fixes or did you just happen to notice this?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.27%. Comparing base (67bee55) to head (ca0af15).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
rpc_util.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8033      +/-   ##
==========================================
+ Coverage   82.22%   82.27%   +0.04%     
==========================================
  Files         383      382       -1     
  Lines       38688    38711      +23     
==========================================
+ Hits        31813    31848      +35     
+ Misses       5555     5547       -8     
+ Partials     1320     1316       -4     

Files with missing lines	Coverage Δ
rpc_util.go	80.67% <0.00%> (-0.22%)	⬇️

... and 30 files with indirect coverage changes

[arjan-bal]

Is there an open issue that this fixes or did you just happen to notice this?

For debugging #8023, I changed the type of BufferSlice from a slice to a struct. I was storing stack traces along with the BufSlice objects. This broke the build and I spotted this issue when fixing all the places that relied on BufSlice being a slice.