stats/opentelemetry: dynamically calculate compressed message size in tests

[eshitachandwani]

Tests in stats/opentelemetry and stats/opentelemetry/csm asserted a hardcoded compressed message size of 57 bytes for a 10,000-byte zero payload with gzip compression.
In Go 1.27, standard library compress/gzip compression improvements changed the output size of this payload from 57 bytes to 54 bytes, causing metric and trace size assertions to fail when executed under newer Go toolchains.

Added GzipCompressedMessageSize helper in to compute the expected compressed byte length dynamically via compress/gzip. Updated metric and trace assertions to use the same.

RELEASE NOTES: None

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.40%. Comparing base (965a6bf) to head (5157154).

Files with missing lines	Patch %	Lines
...tats/opentelemetry/internal/testutils/testutils.go	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9346      +/-   ##
==========================================
+ Coverage   83.15%   87.40%   +4.25%     
==========================================
  Files         424      424              
  Lines       35312    30284    -5028     
==========================================
- Hits        29362    26470    -2892     
+ Misses       4436     3814     -622     
+ Partials     1514        0    -1514     

Files with missing lines	Coverage Δ
...tats/opentelemetry/internal/testutils/testutils.go	97.03% <72.72%> (+1.65%)	⬆️

... and 423 files with indirect coverage changes

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[mbissa]

/gemini review

[gemini-code-assist]

Code Review

This pull request replaces hardcoded gzip-compressed message sizes (previously set to 57 bytes) in CSM and end-to-end tests with dynamically calculated sizes. A new helper function 'GzipCompressedMessageSize' is introduced in the internal 'testutils' package to marshal and gzip-compress protobuf messages to determine their exact size dynamically. There are no review comments, and we have no feedback to provide.

[mbissa]

Please remove this.

[mbissa]

LGTM, modulo tests passing.