fix(bigtable): async context propagation for tracing

[dbolduc]

Fixes #12880, a similar PR to #12921, which was for updating storage. This one is for bigtable.

Generally, we need to replace any opentelemetry::trace::Scope with g::c::i::OTelScope. And if we are not using EndSpan(..., future<T>), be sure to manually DetachOTelContext(...) when the operation is over.

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[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (37619f6) 92.99% compared to head (0a288f4) 92.99%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
.../bigtable/internal/data_tracing_connection_test.cc	98.36%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13156   +/-   ##
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  Coverage   92.99%   92.99%           
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  Files        2137     2137           
  Lines      185828   185855   +27     
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+ Hits       172815   172842   +27     
  Misses      13013    13013           

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