core: change metadata::Kind to a bitflag

[hawkw]

This changes the Kind type to a bitflag, in order to represent
callsites that are hints but also count as spans or events. The goal
here is to allow variants of the enabled! macro that specifically
check if a span would be enabled, or if an event would be enabled.

See this comment for details.

This does not actually implement the enabled! variants, just the
Kind representation change. This way, we can add to the enabled!
macro in a subsequent tracing release without having to change
tracing-core again.

I went with the bitflag representation rather than adding a bool to the
KindInner::Span and KindInner::Event enum variants because it felt a
bit simpler and maybe more performant, although I don't think it's
particularly important to micro-optimize here. I'd consider changing
this to an enum-based representation if people think it's significantly
easier to understand.

[guswynn]

this is awesome, thanks!

[kornelski]

This broke tokio-rs/console#270

[nagisa]

This also broke standard-ai/tracing-gstreamer#6. I’ll be fixing this on my side but I wanted to make a note and give a heads-up:

In tokio-rs/console#270 (comment) @hawkw wrote:

In this case, I am fairly sure that the console is approximately the only code that matched on this type, since there should be no publicly accessible way for user code to actually get a Kind from a Metadata

While there is no public way to obtain Kind from a Metadata, nothing prevented code from doing something along the lines of spelling out Kind::EVENT to construct a Kind and then matching on that value later.

That said, I'm quite sympathetic to the fact that the exhaustiveness was unintentional.

[hawkw]

@nagisa Ugh, sorry about that. I went back and forth for a while on whether this release should be yanked, and I was hoping that the blast radius was small enough that not yanking it was an acceptable decision, because exhaustive matching was never intended here.

I hope this didn't cause you too much of an inconvenience!

[nagisa]

tracing-gstreamer in particular is not very widely used and doesn't expose Kind in its public API making it fairly straightforward to address it on my side. I'm not sure if there will be any other affected crates, but tracing-gstreamer is quite unconventional implementation of a subscriber, so hopefully there won't be any!