Investigate use of seq_trace label for span context

Add Vince to assignees when he accepts his invite to the repo.

From the meeting minutes:

- Consider using `seq_trace` to support tracing independent of library (The main issue is that it could interfere with other uses of the tracing token)
- http://erlang.org/doc/man/seq_trace.html
- In terms of logger metadata, we should plan to use OTP21 logger, which is planned to be integrated with Elixir in 1.10 or 1.11 (2020)
- Ideally, the `seq_trace` token would be a map, so that multiple use-cases could share it using different keys (i.e. users would get_token, update the map, and set_token)
- Perhaps the initial focus should be on explicit white-box tracing?
- `seq_trace` only propagates context between processes via message passing - creation of spans is still as normal and not magical/automatic.
- Action: Vince & Greg to look into whether `seq_trace` is going to be usable for us before the next meeting.