Introduce UserAgent field to allow overriding the default gRPC user-agent string.

[57Ajay]

fixes #14686

Problem

When a user wants to override the User-Agent sent by the otlp (gRPC) exporter,
there is no way to do so. The downstream server always receives the builtin
BuildInfo-derived agent string (e.g. opentelemetry-collector/0.145.0 (linux/amd64)).

This is because exporter/otlpexporter/otlp.go unconditionally passes the builtin
agent to grpc.WithUserAgent() as a dial option:

agentOpt := configgrpc.WithGrpcDialOption(grpc.WithUserAgent(e.userAgent))

grpc.WithUserAgent() sets the User-Agent at the HTTP/2 transport level, which
takes precedence over any per-call metadata headers — so even if a user sets
User-Agent via headers:, the transport-level value wins and the user's value
is silently discarded.

Fix

Following review feedback from @bogdandrutu and @dmathieu, instead of
fixing this only inside otlpexporter, the fix is applied universally in
configgrpc.ClientConfig so that all gRPC components benefit:

1. New UserAgent field on ClientConfig (mapstructure:"user_agent,omitempty")
   — a dedicated, first-class config knob for overriding the default gRPC user-agent.

2. Wiring in getGrpcDialOptions — when cc.UserAgent is set, a
   grpc.WithUserAgent(cc.UserAgent) dial option is appended after extraOpts.
   This ensures it takes precedence over the default BuildInfo-derived string that
   otlpexporter passes via extraOpts, since gRPC-Go applies dial options
   sequentially (last WithUserAgent wins).

3. No changes to otlp.go — the exporter continues passing the BuildInfo
   string as the default via extraOpts. When a user sets user_agent: in their
   config, configgrpc appends it last and it naturally overrides the default.

Usage

exporters:
  otlp:
    endpoint: "0.0.0.0:4444"
    user_agent: "My Distribution For The Collector"
    tls:
      insecure: true

Testing

• TestGrpcClientUserAgent in configgrpc_test.go — verifies that setting
  UserAgent on ClientConfig produces the expected dial option in
  getGrpcDialOptions.

• TestUserAgentHeader_OverriddenByConfig in otlp_test.go — end-to-end test
  that starts a real gRPC server, configures the exporter with a custom UserAgent,
  sends metrics, and asserts the custom value reaches the server while the builtin
  BuildInfo agent string does not.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.39%. Comparing base (1cf90bc) to head (d2fe202).
⚠️ Report is 157 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14699      +/-   ##
==========================================
- Coverage   91.85%   91.39%   -0.46%     
==========================================
  Files         685      695      +10     
  Lines       43391    44392    +1001     
==========================================
+ Hits        39855    40572     +717     
- Misses       2459     2687     +228     
- Partials     1077     1133      +56     

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[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 7 untouched benchmarks
⏩ 76 skipped benchmarks¹

---

_{Comparing 57Ajay:fix/otlp-grpc-user-agent-override (d2fe202) with main (7cd8f58)}

Footnotes

1. 76 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[57Ajay]

/rerun

[57Ajay]

/rerun

[57Ajay]

/rerun

[XuechunHou]

@bogdandrutu A gentle ping to review and approve the PR, thanks

[XuechunHou]

Can this be merged please?

[mx-psi]

I will merge this by EOW unless there are blocking comments by then cc @bogdandrutu

[bogdandrutu]

I am confused about the title of the PR, it does not fix anything "broken" since this config did not exists, and also it applies to all exportes.

[braydonk]

I think the title is just out of date compared to feedback on the solution; initial pitch iirc was to have this be set via the User-Agent header in the headers config (which is actually ignored, and the PR was originally "fixing" that), but the idea pivoted after Damien's comment on the issue to the current state of using a config field.

I'd edit the title myself but I don't have the permissions on this repo. @57Ajay I would suggest adjusting the title to basically what you have in the changelog here - the component is actually [pkg/config/configgrpc] and the title should say what's in the note section. (Or perhaps someone with permissions on the repo can make that adjustment, I think it's non-controversial)

[otelbot]

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