Check if otelgrpc metrics are enabled

[dashpole]

Part of open-telemetry/opentelemetry-go#7800

The only synchronous metric is the duration histogram. This updates it to check whether it is enabled before computing attributes or recording observations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.5%. Comparing base (97de447) to head (aa14c90).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8792   +/-   ##
=====================================
  Coverage   83.5%   83.5%           
=====================================
  Files        192     192           
  Lines      15630   15637    +7     
=====================================
+ Hits       13061   13068    +7     
  Misses      2121    2121           
  Partials     448     448           

Files with missing lines	Coverage Δ
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	96.6% <100.0%> (+<0.1%)	⬆️

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