Add tracing to PubSub publishing

[hexedpackets]

OpenCensus instrumentation for PubSub was just released. This would be really nice to have - we have tracing for the consumers in Heroic already, so having the publisher traced would give us full end-to-end visibility.

There are other parts of the system (core/filtering and ingestion) that would also be nice to have, but getting the publisher traced first would be the most useful IMO.