feat: allow OpenTelemetry context access from SpanRef

[bantonsson]

Motivation

Since OtelData is no longer public, there needs to be a way to access the OpenTelemetry context from other Layers and ensure that the context is created.

Solution

Introduce a SpanRefExt that given a SpanRef and the Dispatch for the layers will get or create the OpenTelemetry context.

Note

Both the test and the example has a workaround for tokio-rs/tracing#3379 which makes the code unnecessarily complicated.

[mladedav]

I'll have a closer look later, but on first look, I have some reservations. But maybe they'll be solvable in this design.

The first thing I noticed when skimming this is that SpanRefExt::context should definitely take &mut self because it internally takes an exclusive lock on its extensions which would definitely lead to people deadlocking themselves through that.

I don't think this makes sense to rush this now so I'll leave it out of today's release and go with the simpler version that does not start the context automatically.

[bantonsson]

Thanks for noticing @mladedav. That is definitely a problem. No need to rush it.

I don't think that a &mut self will protect against the locking issue. I think that the ExtensionMut needs to be passed into the function to make it explicit.

[mladedav]

You're right, for some reason I though when you have ExtensionMut it mutably borrows the SpanRef but it's just normal borrow (and on top of that you can just get another SpanRef by callign ctx.span(&span.id)).

Passing the extensions explicitly is also probably better since the user will most likely already have the lock or will want to get it later.