otelgrpc: add WithSpanKind option

[rauanmayemir]

Summary

• Add WithSpanKind option to override the default span kind for handlers

Details

This adds a WithSpanKind option to NewServerHandler and NewClientHandler that allows overriding the default span kind.

By default:

• NewServerHandler creates spans with SpanKindServer
• NewClientHandler creates spans with SpanKindClient

Use Case

When gRPC is used behind HTTP-to-gRPC transcoding layers (e.g., grpc-gateway, vanguard-go), the outer HTTP handler already creates a SERVER span. The inner gRPC handler creating another SERVER span results in duplicate service boundaries in trace visualizations.

Using WithSpanKind(trace.SpanKindInternal) allows marking these internal protocol translations correctly.

Example

grpc.StatsHandler(otelgrpc.NewServerHandler(
    otelgrpc.WithSpanKind(trace.SpanKindInternal),
))

Fixes #8490

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rauanmayemir / name: Rauan Mayemir (0cf81f1, 42896b7)

[dmathieu]

Please sign the CLA

[rauanmayemir]

Should I squash and force push?

[dmathieu]

No. We'll auto-squash when merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.2%. Comparing base (82cfc18) to head (0cf81f1).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8506   +/-   ##
=====================================
  Coverage   82.2%   82.2%           
=====================================
  Files        179     179           
  Lines      13692   13700    +8     
=====================================
+ Hits       11257   11265    +8     
  Misses      2033    2033           
  Partials     402     402           

Files with missing lines	Coverage Δ
...entation/google.golang.org/grpc/otelgrpc/config.go	87.8% <100.0%> (+0.7%)	⬆️
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	98.4% <100.0%> (+<0.1%)	⬆️

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[rauanmayemir]

CLA is signed, changes are approved.
When will this be merged? (is it automatic)

@dmathieu blocked label is still there.

[dmathieu]

We require for PRs to be open for at least 24h before they are merged.

[flc1125]

Sorry, although it has been merged, we might need to make some adjustments. I checked the semantic conventions for gRPC, and I think for Server or Client, their corresponding SpanKind must only be SpanKindServer and SpanKindClient, respectively.

Reference: https://opentelemetry.io/docs/specs/semconv/rpc/grpc/#client-span

This means we must enforce the definition of the corresponding Server and Client and remove the WithSpanKind() method.

[dmathieu]

Did you see the reasoning in #8490?

[flc1125]

Did you see the reasoning in #8490?

Sorry, I missed it. Currently, this PR seems reasonable.

[rauanmayemir]

Had me worried for a second there. 😄