impl(otel): detach context from EndSpan(..., future<>)

[dbolduc]

Part of the work for #12880

These EndSpan(..., future<>) are mainly a .then() which might execute in a different thread than the future is returned in. Detach the context from the original thread in the callback thread.

This change is currently a no-op until we start pushing/popping the context and copying the OTelContext into the callback threads.

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This change is 

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (9523270) 93.61% compared to head (8bb88e7) 93.61%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12902   +/-   ##
=======================================
  Coverage   93.61%   93.61%           
=======================================
  Files        2067     2067           
  Lines      180045   180086   +41     
=======================================
+ Hits       168546   168588   +42     
+ Misses      11499    11498    -1     

Files	Coverage Δ
google/cloud/internal/grpc_opentelemetry.h	94.11% <100.00%> (+1.26%)	⬆️
google/cloud/internal/grpc_opentelemetry_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/opentelemetry.h	100.00% <100.00%> (ø)
google/cloud/internal/opentelemetry_test.cc	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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