impl(otel): trace context across AsyncPollingLoop

[dbolduc]

Part of the work for #10618

Propagate trace context across an Async(Rest)PollingLoop

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This change is 

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f81ff8b) 93.75% compared to head (a9807ff) 93.76%.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #11040    +/-   ##
========================================
  Coverage   93.75%   93.76%            
========================================
  Files        1725     1725            
  Lines      155440   155535    +95     
========================================
+ Hits       145739   145839   +100     
+ Misses       9701     9696     -5     

Impacted Files	Coverage Δ
google/cloud/internal/async_polling_loop.cc	100.00% <100.00%> (ø)
google/cloud/internal/async_polling_loop_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/async_rest_polling_loop.cc	100.00% <100.00%> (ø)
...gle/cloud/internal/async_rest_polling_loop_test.cc	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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