Link tracing between open stream and read range.

**What component of `google-cloud-cpp` is this feature request for?** For
GCS

**Is your feature request related to a problem? Please describe.** A clear and
So far, we have spans for Open stream in [ConnectionTracing](https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/storage/internal/async/connection_tracing.cc#L52) and for ReadRange operation in [ObjectDescriptorReaderTracing](https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/storage/internal/async/object_descriptor_reader_tracing.cc) . But we need to link them so that we know the objects being read in the same stream.