Open census call attempt span name and attribute changes

[yashykt]

Updates based on https://github.com/grpc/proposal/pull/254/files

1. Per-attempt span name - "Attempt.<service_name>.<method_name>"
2. Add two attributes to the per-attempt span -
   i) previous-rpc-attempts - an integer value, number of preceding attempts, transparent retry not included.
   ii) is-transparent-retry - a boolean value, whether the attempt is a transparent retry.

[markdroth]

Looks great -- just a couple of minor nits.

[markdroth]

Prior to this PR, this assert would be run before passing the fields from parent_->context_ to the CensusContext ctor, but now it's being done after. Is that okay?

[yashykt]

moved to StartNewAttempt

[markdroth]

That's not quite optimal, since someone could later change the code to instantiate OpenCensusCallAttemptTracer from somewhere else, and they might not remember to add the assertion.

Is CensusContext movable? If so, you can use a helper function like this:

namespace {

CensusContext CreateCensusContextForCallAttempt(
    absl::string_view method, const CensusContext& parent_context) {
  GPR_DEBUG_ASSERT(parent_context.Context().IsValid());
  return CensusContext(method, &parent_context.Span(), parent_context.tags());
}

}  // namespace

Then in the initializer list, you can do this:

       context_(CreateCensusContextForCallAttempt(
                    absl::StrCat("Attempt.", parent_->method_),
                    parent_->context_)),

[yashykt]

works!

[markdroth]

Thanks for doing this!

Please back-port to 1.40.

[yashykt]

Known issues: #26595, #26885
The interop cloud-to-cloud test timeout seems to be unrelated.