[otelgrpc] Do not panic for nil instruments

[MrAlias]

There is no guarantee when a Meter returns an error the instrument will not be nil. Default to a no-op implementation of the instrument in that case so the instrumentation does not panic.

Close open-telemetry/opentelemetry-go#4836

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ef8063b) 81.3% compared to head (b02d422) 81.5%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4875     +/-   ##
=======================================
+ Coverage   81.3%   81.5%   +0.1%     
=======================================
  Files        151     151             
  Lines      10894   10904     +10     
=======================================
+ Hits        8866    8891     +25     
+ Misses      1869    1859     -10     
+ Partials     159     154      -5     

Files	Coverage Δ
...entation/google.golang.org/grpc/otelgrpc/config.go	87.8% <100.0%> (+18.2%)	⬆️