impl(otel): propagate OTelContext

[dbolduc]

Part of the work for #12880

Reimplement CallContext to use our OTelContext instead of using the active span.

Our current implementation produces traces that look sorta correct. But it has problems, as described in the parent issue. This PR breaks the output that "looks sorta correct".

I talked with @alevenberg and we deemed this approach preferable to sending a mega-PR with GAPIC, handwritten library changes. (As long as the issue gets resolved by the next release.)

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We remove the "call span active throughout" tests in the async retry loops because it is not something the retry class is doing. That is a property of the async operations, e.g. AsyncUnaryRpcFuture.

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Update ABI dumps because we have a false positive on missing templates for types that we don't even own:

• GCB: https://console.cloud.google.com/cloud-build/builds;region=us-east1/af98c1a5-740d-42b3-a75e-012836f6c0b5;tab=detail?project=cloud-cpp-testing-resources
• Raw: https://storage.googleapis.com/cloud-cpp-community-publiclogs/logs/google-cloud-cpp/12915/2717b3e12135ba0ac8af4c7356cfea6a21d20d23/fedora-latest-cmake-check-api/log-af98c1a5-740d-42b3-a75e-012836f6c0b5.txt

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timestamp.h
namespace opentelemetry::v1::common
SystemTimestamp::SystemTimestamp<long, std::ratio<1l, 1000000000l> > [C4] 
( std::chrono::duration<long, std::ratio<1l, 1000000000l> >const& 
time_since_epoch )
_ZN13opentelemetry2v16common15SystemTimestampC4IlSt5ratioILl1ELl1000000000EEEERKNSt6chrono8durationIT_T0_EE



utility.h
namespace absl
forward<variant<monostate>&> ( remove_reference_t& t )
_ZN4absl7forwardIRNS_7variantIJNS_9monostateEblmdN13opentelemetry2v15nostd10shared_ptrINS4_5trace4SpanEEENS6_INS7_11SpanContextEEENS6_INS4_7baggage7BaggageEEEEEEEEOT_RNSt16remove_referenceISH_E4typeE



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Header Files  82 

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This change is 

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (37f726c) 93.61% compared to head (771ca4d) 93.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12915      +/-   ##
==========================================
- Coverage   93.61%   93.61%   -0.01%     
==========================================
  Files        2067     2067              
  Lines      180140   180104      -36     
==========================================
- Hits       168642   168602      -40     
- Misses      11498    11502       +4     

Files	Coverage Δ
...e/cloud/bigtable/internal/async_bulk_apply_test.cc	97.51% <100.00%> (ø)
...e/cloud/bigtable/internal/async_row_reader_test.cc	97.47% <100.00%> (ø)
.../cloud/bigtable/internal/async_row_sampler_test.cc	97.68% <100.00%> (ø)
google/cloud/bigtable/row_reader_test.cc	98.21% <100.00%> (ø)
...ogle/cloud/internal/async_connection_ready_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/async_polling_loop_test.cc	100.00% <100.00%> (ø)
...loud/internal/async_read_write_stream_impl_test.cc	97.94% <100.00%> (ø)
...gle/cloud/internal/async_rest_polling_loop_test.cc	99.04% <100.00%> (ø)
...oogle/cloud/internal/async_rest_retry_loop_test.cc	98.33% <100.00%> (-0.07%)	⬇️
google/cloud/internal/async_retry_loop_test.cc	98.85% <100.00%> (-0.05%)	⬇️
... and 5 more

... and 5 files with indirect coverage changes

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