Respect events' parent in tracing-opentelemetry

[wprzytula]

Bug Report

Version

tracing-opentelemetry = "0.17.4"

Platform

[Irrelevant]

Crates

tracing-opentelemetry

Description

When creating events in asynchronous code, it is valid to add one to a span that is not current (right after awaiting the instrumented future - see below). However, OpenTelemetryLayer::on_event() uses current span for adding events no matter if the event has its parent explicitly set.

This code:

let outer_span = trace_span!("Outer");
async {
	let inner_span = trace_span!("Inner");
	some_async_function()
		.instrument(inner_span.clone())
		.await
		.map_err(|err| {
			error!(parent: &inner_span, "Error occured!");
	}.instrument(outer_span).await;

results in "Error occured!" being attached to outer span instead of inner span.

Proposed solution

Change OpenTelemetryLayer::on_event() to respect event's parent if set, falling back to current span if not set.