impl(otel): traced async backoff in retry loop

[dbolduc]

Part of the work for #10618

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This change is 

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fa66623) 93.75% compared to head (86850a4) 93.75%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11051   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files        1727     1727           
  Lines      155723   155745   +22     
=======================================
+ Hits       145997   146021   +24     
+ Misses       9726     9724    -2     

Impacted Files	Coverage Δ
google/cloud/internal/async_rest_retry_loop.h	100.00% <100.00%> (ø)
...oogle/cloud/internal/async_rest_retry_loop_test.cc	99.24% <100.00%> (+0.02%)	⬆️
google/cloud/internal/async_retry_loop.h	100.00% <100.00%> (ø)
google/cloud/internal/async_retry_loop_test.cc	99.49% <100.00%> (+0.01%)	⬆️

... and 6 files with indirect coverage changes

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