Add semantic conventions for "Process" spans for messaging scenarios

[pyohannes]

Currently, the messaging workgroup works on an open-telemetry/oteps#220 which covers span structures in messaging scenarios.

The group decided to focus on a consistent set of conventions that can be applied across all messaging scenarios, which resulted into a proposal for conventions for "Publish", "Create", "Deliver", "Receive", and "Settle" operations, as those share common characteristics across all messaging scenarios. The same can't be said of "Process" operations, which can vary considerable depending on the individual use-case.

However, as interest was expressed from many sides to also achieve some consistency for the instrumentation of "Process" operations, it is necessary to provide conventions for "Process" operations as an addition to what's already in open-telemetry/oteps#220.

This work doesn't block merging open-telemetry/oteps#220.

[Oberon00]

See esp. thread at open-telemetry/oteps#220 (comment)

[pyohannes]

Discussed in today's workgroup meeting: we want to define "process" spans, but it's not the goal to stabilize those together with messaging semantic conventions. We will work towards a "mixed" stability, where "process" and "settle" spans are experimental, but other parts of the semantic conventions are stable.

The motivation behind is to not delay stabilization efforts with discussions around details, while still having confidence that we can stabilize "process" and "settle" spans without breaking other parts.