Span control in MakeWriter<'_>

[feelingnothing]

Feature Request

Crates

tracing-subcriber

Motivation

For the moment only metadata available in the MakeWriter is event's metadata, which is very limited. A basic use-case of creating different writers for each span with certain value specified is impossible.
I don't think my use-case is very niche. I have a tonic service that creates workers, each with unique id. I want to store a log for each bot separate.
The most logical solution for that came to mind is to create a worker-wide span with its id in the span and open/drop (a handle) based on span's lifetime. Create/open a file and write to it.
Now this is a motivation part. There is no API to either get access to span's data at all.

Proposal

Add on_span_enter, on_exit and Context<'_, S> to the MakeWriter trait.

Alternatives

Can't find one, FmtContext's constructor being behind private does not let me format my own fields in the record.

Comments

This is my code that's trying to circumvent this: https://gist.github.com/feelingnothing/2db9cc4734aece2d3fdaa2601d53b8a8