Allow user use custom grpc::ChannelArguments

[owent]

Fixes #3883

Changes

• Allow users to use custom grpc::ChannelArguments with SDK.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[Copilot]

Pull request overview

This PR extends the OTLP gRPC exporter/client configuration to allow callers to supply custom grpc::ChannelArguments, enabling advanced gRPC tuning (e.g., disabling HTTP proxy handling to avoid getenv-related races) while preserving existing option-derived defaults.

Changes:

• Add channel_arguments to OtlpGrpcClientOptions and copy caller-provided arguments into the channel configuration.
• Refactor channel-argument construction into OtlpGrpcClient::BuildChannelArguments() and reuse it from MakeChannel().
• Add a unit test validating that caller-provided channel arguments are copied and that user-agent configuration is applied.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_client_options.h	Adds a new options field for passing custom gRPC channel arguments.
exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_client.h	Adds an internal helper for building channel arguments (used by implementation + tests).
exporters/otlp/src/otlp_grpc_client.cc	Implements copying of caller-provided channel arguments and refactors channel argument construction.
exporters/otlp/test/otlp_grpc_target_test.cc	Adds coverage to verify channel-argument copying and user-agent behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (0962d71) to head (aaa63bc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3990      +/-   ##
==========================================
+ Coverage   90.32%   90.33%   +0.02%     
==========================================
  Files         230      230              
  Lines        7299     7299              
==========================================
+ Hits         6592     6593       +1     
+ Misses        707      706       -1     

see 1 file with indirect coverage changes

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[ThomsonTan]

This seems redundant because it is already default initialized.

[lalitb]

nit - Question: can we make the lifetime requirement a bit more explicit here? Something like:

/*
This is a non-owning pointer; the pointed-to grpc::ChannelArguments must remain valid until client/channel construction copies it.
*/

Currently, "Copied when the channel is created" is directionally right, but it still leaves the required lifetime a little ambiguous.

[lalitb]

LGTM. nit comments.