fmt/writer: relax lifetime constrain of metadata

[BiagioFesta]

Motivation

tracing::Event seems quite stable to return a &'static Metadata<'static>; thus, in the MakeWriter we could relax lifetime constrains so it would be easier to implement aSelf::Writer which requires a reference to the meta

[hawkw]

Metadata for events produced by the tracing-log adapter cannot have the 'static lifetime, as log::Records have a generic lifetime. Therefore, this change would make tracing_subscriber::fmt incompatible with tracing-log.

Unfortunately, I don't think we can make this change for that reason. Sorry! Depending on what you need &'static Metadata for, there might be another solution where we can provide the make_writer_for with static metadata when it is coming from a tracing macro via a separate method, with a default implementation that calls into the generic-lifetime version.

[BiagioFesta]

Thanks for the clarification! Yeah, I probably should have checked the tracing-log integration first before submitting the PR. The change seemed straightforward so I thought I'd open a PR to get some initial feedback.

I'm currently looking at the code from my phone while on the train (so I might be missing something :D), but doesn't dispatch_record end up creating a static metadata anyway?

In any case, what I'm actually trying to do is create a Writer that can hold a reference to the event's metadata