End-to-end test for Trace Propagation

We implemented trace propagation in #4667 ; the only service I could find that supports server-side spans is Cloud Spanner using [end to end tracing](https://docs.cloud.google.com/spanner/docs/set-up-tracing#configure-end-to-end-tracing). This is pretty cool since we can direct spanner to sample a trace from our integration tests.

This depends on the spanner client library becoming pub and implementing the feature linked, so we can't build it right now. When Spanner is a bit further along, we should be able to write a neat integration test that makes sure the trace ID is propagated correctly in a format that the Google backend understands.