Implement follows_from

[hawkw]

As per tokio-rs/tokio#561:

Besides just parent / child relationships, there is also a need to track more complex causality. One example is the h2 crate. The HTTP 2.0 protocol allows request / response exchanges to be multiplexed on a single TCP socket. To implement this, a dedicated task is spawned to manage the TCP socket. Each HTTP 2.0 request / response exchange interacts with the TCP socket task via message passing. Because of this structure, there is no clear parent / child relationship between the logic processing the request / response exchange and the logic that does the actual TCP work for that request / response exchange. However, we still want to track the causality to aid in debugging.

To support this goal, events may have an arbitrary number of follows_from annotations. This creates a relationship between two events from separate traces.

[hawkw]

This is done for spans. I still need to figure out a good API for events...

[hawkw]

The changes necessary to make this usable for Events are in the event! macro, rather than in core; it's possible to construct Events which follow from spans using the core API. Thus, I'm going to close this issue & open a new ticket to track making the event macro support this.