PubSub add OpenTelemetry support

[sethmaxwl]

OpenTelemetry should be added to PubSub to provide OpenTracing and OpenCensus functionality as one API and SDK. It would be helpful to use it to trace a message throughout its life cycle.

Update: Requires #131. The latter will drop Python 2 support, which is a prerequisite, since the OpenTelemetry library requires Python 3.

[plamut]

Just as a quick status update, it is still not clear what exactly do want to be tracked, still awaiting the official specs. We can thus consider this blocked until further notice, and the pull request should not be merged yet.

[acocuzzo]

Closing this as a duplicate of: #543