Add all fields to span name

[superdump]

I'm not sure if this is desirable in its current form, but this is super useful for me. I'm open to input for how to improve it.

[thoren-d]

Thanks for the contribution!

I was thinking I wanted to add fields at some point and I like that using FormattedFields makes this code simpler.

Is it important that the field values be in the name of each span for your application? I was thinking we could put them in the "args" object for each span enter and exit event. This is where I put the file and line number for each span.

Also, an option to enable/disable including fields would be good, as they can make the resulting file much larger.

[superdump]

I was motivated to make this PR because of a problem with naming Spans in the bevy game engine. It schedules a bunch of functions to run at different times based on structs that are derived for functions. These structs contain the name of the function, obtained by std::any::type_name<T>() -> &'static str where T is the type of the function, and it's stored in a Cow<'static, str>. As tracing Span names must be constants (it seems), I have had to add the name of the function being executed as a field on the span. That then leads to this PR because a lot of the interesting Spans added to bevy's code have a name field and visualising the traces is only really useful by adding the field into the name, otherwise all the spans are called 'system' or 'stage'. So, I'm trying to see if there's a compile-time way to automatically instrument all systems.

[thoren-d]

@superdump I've just published 0.3.0 with a new name_fn option which should enable your use case. Let me know if it works for you.

[superdump]

Works like a charm! Thanks!