[OTLP] Add support for GZip compression

[martincostello]

Fixes #3961

Changes

Add support for GZip compression to the OTLP exporter.

Continues the work started in #6494.

Once released, should also update the compliance matrix.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.16%. Comparing base (7dd46f4) to head (3b9d5c8).
⚠️ Report is 14 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7055      +/-   ##
==========================================
+ Coverage   89.09%   89.16%   +0.07%     
==========================================
  Files         271      271              
  Lines       13065    13120      +55     
==========================================
+ Hits        11640    11699      +59     
+ Misses       1425     1421       -4     

Flag	Coverage Δ
unittests-Project-Experimental	88.87% <100.00%> (-0.21%)	⬇️
unittests-Project-Stable	88.90% <100.00%> (-0.13%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ol/Implementation/ExportClient/OtlpExportClient.cs	100.00% <100.00%> (ø)
...mplementation/ExportClient/OtlpGrpcExportClient.cs	78.94% <100.00%> (+4.21%)	⬆️
...mplementation/ExportClient/OtlpHttpExportClient.cs	100.00% <100.00%> (ø)
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	99.37% <100.00%> (+0.09%)	⬆️

... and 6 files with indirect coverage changes

[Copilot]

Pull request overview

Adds configurable gzip compression support to the OTLP exporter to align with the OTLP exporter specification (including env var configuration) and validates behavior via new/updated unit tests.

Changes:

• Introduces OtlpExportCompression and OtlpExporterOptions.Compression, with spec env var support (OTEL_EXPORTER_OTLP*_COMPRESSION).
• Implements gzip request compression for OTLP over HTTP and gRPC export clients.
• Adds/updates unit tests and updates changelog + public API tracking files.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpSpecConfigDefinitionTests.cs	Extends spec config test data and assertions to include compression env vars.
test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpHttpExportClientTests.cs	Adds HTTP export tests validating payload and headers with/without gzip.
test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpGrpcExportClientTests.cs	Adds gRPC export tests validating compressed framing and decompression behavior.
test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpExporterOptionsTests.cs	Extends options/config tests to cover compression env var parsing and invalid values.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptions.cs	Adds Compression option, parsing helper, and spec-env-var application.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExportCompression.cs	New public enum defining supported compression modes.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/IOtlpExporterOptions.cs	Adds Compression to internal options contract.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/OtlpSpecConfigDefinitions.cs	Adds spec env var constants for compression across signals.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/ExportClient/OtlpHttpExportClient.cs	Implements gzip compression for HTTP request content.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/ExportClient/OtlpGrpcExportClient.cs	Implements gzip compression for gRPC framing + request header.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/ExportClient/OtlpExportClient.cs	Centralizes content creation via overridable method; wires CompressionEnabled.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md	Documents new opt-in gzip compression configuration.
src/OpenTelemetry.Exporter.OpenTelemetryProtocol/.publicApi/Stable/PublicAPI.Unshipped.txt	Tracks new public API surface (OtlpExportCompression, OtlpExporterOptions.Compression).

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