Optimize otelgrpc, avoid unnecessary allocations and copies

[bogdandrutu]

Depends on #2835

[codecov]

Codecov Report

Merging #2838 (1d598cb) into main (1b75ca9) will decrease coverage by 0.1%.
The diff coverage is 84.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2838     +/-   ##
=======================================
- Coverage   69.4%   69.2%   -0.2%     
=======================================
  Files        145     145             
  Lines       6712    6709      -3     
=======================================
- Hits        4660    4648     -12     
- Misses      1938    1947      +9     
  Partials     114     114             

Impacted Files	Coverage Δ
...ation/google.golang.org/grpc/otelgrpc/grpctrace.go	45.7% <78.9%> (-3.5%)	⬇️
...ion/google.golang.org/grpc/otelgrpc/interceptor.go	83.4% <100.0%> (-0.8%)	⬇️