feat: OpenTelemetry context activation POC

[bantonsson]

TL;DR

This PR is opened as a basis for discussion around providing a more seamless interoperability for users of the OpenTelemetry APIs when using frameworks and libraries that use tracing at their core.

An OpenTelemetry Context that mirrors the active tracing Span is now active on the executing thread while in a tracing Span. Having proper OpenTelemetry context activation will not only make interoperability easier for users of the OpenTelemetry API, but will also open up the possibility for simpler log/trace/baggage correlation and in the future profiling/trace correlation among other things.

Motivation

The aim is to provide users of the OpenTelemetry API a more seamless and consistent experience. One such thing is that calling the OpenTelemetry API Context::current() did not give you a Context that contained an OpenTelemetry Span that represented the current tracing Span. Another thing is propagation of Baggage or user defined types that were not picked up either.

A lot of effort has been put into making the existing OpenTelemetrySpanExt API work the same way to maintain maximum backwards compatibility and to avoid fragmentation by building a separate OpenTelemetry and Tokio Tracing bridge.

Long discussion about OpenTelemetry and Tokio Tracing interoperability, and an issue specifically discussing this POC.

Solution

This solution adds a new feature activate_context that will activate/deactivate an OpenTelemetry Context that mirrors the current tracing Span. This feature is currently on by default, but can be turned off to avoid any performance overhead incurred by the context activation and bookkeeping.

The second big change is that the OpenTelemetry SpanBuilder is lazily consumed and a real OpenTelemetry Span is created when necessary, so there is no longer any need for the PreSampledTracer and the special code in OpenTelemetry that it relies on. This has some implications, like you can no longer set the parent_context after you have entered a Span or called the context method on the Span.

Benchmark numbers are available in this comment, and when the activate_context feature is turned off, the performance stays the same. (There has been one more optimization since the last run there, fixing the regression in the many_events benchmarks)

[djc]

I have been passively maintaining tracing-opentelemetry (mostly by reviewing relatively small PRs) for a few months, but I don't currently have a use case for this crate. Unless some funding for doing this work materializes, I'm not motivated to review larger efforts like this, so I suggest the OpenTelemetry and/or tracing organizations find some other people to move this effort forward.