Allow using OTLP export retrier concurrently

[dmathieu]

This change allows calling the OTLP retrier concurrently, so it can be used with several tracer providers.

Example:

exporter, _ := otlptrace.New(ctx, otlptracegrpc.NewClient())
tp1 := otelsdktrace.NewTracerProvider(otelsdktrace.WithBatcher(exporter))
tp2 := otelsdktrace.NewTracerProvider(otelsdktrace.WithBatcher(exporter))

Closes #3755

[codecov]

Codecov Report

Merging #3756 (001981d) into main (de94faf) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3756   +/-   ##
=====================================
  Coverage   81.6%   81.6%           
=====================================
  Files        166     166           
  Lines      12432   12435    +3     
=====================================
+ Hits       10152   10157    +5     
+ Misses      2065    2063    -2     
  Partials     215     215           

Impacted Files	Coverage Δ
exporters/otlp/internal/retry/retry.go	98.3% <100.0%> (+<0.1%)	⬆️
sdk/trace/batch_span_processor.go	81.9% <0.0%> (+0.8%)	⬆️