opentelemetry: fix on_event not respecting events' explicit parents (…

…#2296)

## Motivation

One can want to have an event added from outside context of a span,
especially in an async code (e.g. between polls of the future, or
between polling multiple futures instrumented with that span). Then it
is expected that the event will be attached indeed to the span specified
as the parent and not the contextual one.

Fixes: #2295 

## Solution

See #2295

tracing-opentelemetry/src/subscriber.rs

@@ -620,8 +620,8 @@ where
     /// [`ERROR`]: tracing::Level::ERROR
     /// [`Error`]: opentelemetry::trace::StatusCode::Error
     fn on_event(&self, event: &Event<'_>, ctx: Context<'_, C>) {
-        // Ignore events that are not in the context of a span
-        if let Some(span) = ctx.lookup_current() {
+        // Ignore events that have no explicit parent set *and* are not in the context of a span
+        if let Some(span) = ctx.event_span(event) {
             // Performing read operations before getting a write lock to avoid a deadlock
             // See https://github.com/tokio-rs/tracing/issues/763
             #[cfg(feature = "tracing-log")]