Change visibility of `OtelData` to `pub`

[ollyswanson]

Feature Request

Motivation

A recently merged PR #202 has changed the visibility of OtelData. It was previously pub and it was possible to access it from other layers by poking around in the span extensions - I've been using this to inject trace_id and span_id into my logs.

This is also being used by the opentelemetry-appender-tracing crate for, I think, the same purpose.

Proposal

Make OtelData pub again as it was previously.

I think it might be simpler to just make it pub again as I can't discern any motivation for changing the visibility. I'm happy to raise a PR for it but I want to understand if there was a motivation for the change before doing that.

Alternatives

If there's a better way of doing this I'd be happy to adopt that pattern instead, but up until now it's been a very convenient way to transparently include the trace_id and span_id in my own logging layer.

[mladedav]

Thanks for catching this.

I'm pretty sure we wouldn't want to make it structurally public (it was bad enough before, now unexpected changes to the state would probably be even worse), but we should definitely expose the IDs through a method.

I don't think there really is another way because for example tracing events can be emitted even when their parent span is not entered so this should not rely on the active opentelemetry context.

[ollyswanson]

Exposing them through a method would be lovely - I'm up to my eyeballs with work at the moment but I could try to get a PR in in the next couple of weeks.

Don't consider that to be me asking to squat on it though if someone else gets there first it's one less thing to have to worry about 😄