[configgrpc] Guard OpenTelemetry gRPC instrumentation with a feature gate

[mx-psi]

Description

Guards otelgrpc instrumentation behind a new configgrpc.useOtelGRPCInstrumentation feature flag.

These are unstable semantic conventions being actively worked on by the RPC Semconv SIG (RPC Semantic Conventions Stability) and the otelgrpc library itself is unstable.

I argue this would be beneficial because:

1. It would minimize disruption from changes in RPC conventions
2. It would postpone decisions regarding Review sequence for HTTP and gRPC middleware #12836 and whether to view instrumentation as middleware

[codspeed-hq]

CodSpeed Performance Report

Merging #14222 will not alter performance

_{Comparing mx-psi:mx-psi/configgrpc-feature-gate-otelgrpc (aa8be01) with main (974da01)}

⚠️ 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.

Summary

✅ 71 untouched

[mx-psi]

@open-telemetry/semconv-rpc-approvers I would love to have your opinion on this PR.

My interpretation of https://opentelemetry.io/blog/2025/stability-proposal-announcement/#instrumentation-and-convention-goals is that this would be fine to have enabled by default, but if you believe it is still early for users to adopt these I would like to hear it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.14%. Comparing base (974da01) to head (aa8be01).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14222      +/-   ##
==========================================
- Coverage   92.15%   92.14%   -0.01%     
==========================================
  Files         667      667              
  Lines       41446    41449       +3     
==========================================
  Hits        38194    38194              
- Misses       2215     2217       +2     
- Partials     1037     1038       +1     

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[trask]

@open-telemetry/semconv-rpc-approvers I would love to have your opinion on this PR.

as long as you're following https://github.com/open-telemetry/semantic-conventions/blob/main/docs/rpc/README.md#semantic-conventions-for-rpc

Important

Existing RPC instrumentations that are using
v1.37.0 of this document
(or prior):

• SHOULD NOT change the version of the RPC conventions that they emit by
  default in their existing major version. Conventions include (but are not
  limited to) attributes, metric and span names, and unit of measure.
• SHOULD introduce an environment variable OTEL_SEMCONV_STABILITY_OPT_IN
  in their existing major version as a comma-separated list of category-specific values
  (e.g., http, databases, rpc). The list of values includes:
  • rpc - emit the stable RPC conventions, and stop emitting
    the experimental RPC conventions that the instrumentation emitted
    previously.
  • rpc/dup - emit both the experimental and stable RPC conventions,
    allowing for a phased rollout of the stable semantic conventions.
  • The default behavior (in the absence of one of these values) is to continue
    emitting whatever version of the old experimental RPC conventions
    the instrumentation was emitting previously.
  • Note: rpc/dup has higher precedence than rpc in case both values are present
• SHOULD maintain (security patching at a minimum) their existing major version
  for at least six months after it starts emitting both sets of conventions.
• MAY drop the environment variable in their next major version and emit only
  the stable RPC conventions.