otelgrpc: support OTEL_SEMCONV_STABILITY_OPT_IN

[dashpole]

Fixes #8683

This keeps the default as the new conventions, but supports rpc/dup and rpc/old modes.

The only interesting part is that there is a change to how rpc.method is structured. For the rpc/dup mode, I use the "new" version, rather than the old version.

I used AI to help implement the changes.

[codecov]

Codecov Report

❌ Patch coverage is 94.39252% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.2%. Comparing base (c191508) to head (3b1afee).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	92.5%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8726     +/-   ##
=======================================
+ Coverage   82.1%   82.2%   +0.1%     
=======================================
  Files        183     183             
  Lines      13819   13901     +82     
=======================================
+ Hits       11350   11437     +87     
+ Misses      2065    2056      -9     
- Partials     404     408      +4     

Files with missing lines	Coverage Δ
...entation/google.golang.org/grpc/otelgrpc/config.go	89.8% <100.0%> (+1.9%)	⬆️
.../google.golang.org/grpc/otelgrpc/internal/parse.go	100.0% <100.0%> (ø)
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	96.5% <92.5%> (-2.0%)	⬇️

... and 1 file with indirect coverage changes

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[dmathieu]

Would it make sense to have the semantic convention logic in its own package, like we did for the HTTP packages?

[dashpole]

Would it make sense to have the semantic convention logic in its own package, like we did for the HTTP packages?

I would prefer keeping it in the package. The actual logic to switch between old/new/dup isn't too complicated, IMO. The changes for the rpc migration seem a lot simpler than they were for the http migration, which helps. If we split into a new package, it is just more for us to revert later.