Fix async context propagation

[dbolduc]

Our tracing connection spans for asynchronous operations are still active in the continuations (the .then()s). They should not be. These continuations happen after the operation completes.

auto f = AsyncFoo().then([] (auto f) {
  // The span for "AsyncFoo" is still active here when it should not be.
  return f.get();
});

This is especially problematic when we try to chain multiple asynchronous calls together. They show up as children of each other instead of siblings.

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Googlers can see go/cloud-cxx:otel-async-context-propagation-dd for more details.

• Add OTelContext infrastructure
• Use OTelContext in our CallContext.
• Update the GAPICs
• Update storage handwritten tracing layers
• Update bigtable handwritten tracing layers
• Update pubsub handwritten tracing layers #12958