Unable to set parent context when using #[instrument] macro

[nnmm]

Bug Report

Version

v0.32.1

Platform

Linux x86_64

Description

tracing::Span::current().set_parent(parent_cx) could in the past be used for the receiving end of OTEL trace context propagation, but since 0.32 (specifically c4fe96a) it doesn't work anymore on entered spans.

If I'm not mistaken, that means there is no way to set the parent of a #[instrument] span, because these spans are entered automatically. #[instrument] has a parent argument, but that expects a tracing::Span, not an opentelemetry::Context, so I don't think fits the bill.

#[instrument] is by far the most ergonomic way to create spans for async functions, and it would be very limiting in my opinion to not be able to propagate trace context to these spans.

[mladedav]

As you mentioned, set_parent is used practically just for context propagation, not in most functions. From my experience (propagation in tower services), that span is created explicitly, not through the attribute.

Do you have a use case, where you use the attribute and also want to set the parent explicitly to a context other than the default parent?

[nnmm]

In tonic gRPC services, you might want to use #[instrument] on each individual request handler instead of using Server::trace_fn because it's almost impossible to add request-specific fields to the span created in that function. You might say that we could have both spans (with the trace context propagation happening in Server::trace_fn and the request handler span with the request-specific fields being its child), and that's true, but you pay per span e.g. in Honeycomb and many people prefer few "fat" spans to many "thin" spans anyway.