fix(otel): end async client spans properly

[dbolduc]

Part of the work for #12880

Keep track of the current context in our generated async APIs (and for APIs that return a StreamRange which also needs to restore the original call context before sending RPCs).

This still leaves handwritten traces for async APIs (bigtable, storage, pubsub) looking wonky. They get fixed next.

Have stricter unit tests that verify the new behavior.

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This change is 

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6d50ebb) 93.61% compared to head (8fdf590) 93.61%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12919   +/-   ##
=======================================
  Coverage   93.61%   93.61%           
=======================================
  Files        2067     2067           
  Lines      180104   180116   +12     
=======================================
+ Hits       168601   168624   +23     
+ Misses      11503    11492   -11     

Files	Coverage Δ
...internal/golden_kitchen_sink_tracing_connection.cc	100.00% <100.00%> (ø)
...en/v1/internal/golden_kitchen_sink_tracing_stub.cc	93.51% <100.00%> (ø)
.../internal/golden_thing_admin_tracing_connection.cc	100.00% <100.00%> (ø)
...sts/golden_kitchen_sink_tracing_connection_test.cc	100.00% <100.00%> (ø)
...sts/tests/golden_kitchen_sink_tracing_stub_test.cc	99.57% <100.00%> (+<0.01%)	⬆️
...ests/golden_thing_admin_tracing_connection_test.cc	100.00% <100.00%> (ø)
...ests/tests/golden_thing_admin_tracing_stub_test.cc	100.00% <100.00%> (ø)
generator/internal/tracing_connection_generator.cc	43.90% <ø> (ø)
generator/internal/tracing_stub_generator.cc	62.29% <ø> (ø)
...den/v1/internal/golden_thing_admin_tracing_stub.cc	95.59% <88.88%> (+0.35%)	⬆️

... and 7 files with indirect coverage changes

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