tracing-subscriber: Parent span ref_count + memory gets leaked when using a non-standard Registry type

[Protryon]

Bug Report

Version

tracing-subscriber 0.3.17

Description

Here:

These lines here silently require that the inner type is a Registry in order to run CloseGuard logic in sharded.rs.

#[cfg(all(feature = "registry", feature = "std"))]
let subscriber = &self.inner as &dyn Subscriber;
#[cfg(all(feature = "registry", feature = "std"))]
let mut guard = subscriber
    .downcast_ref::<Registry>()
    .map(|registry| registry.start_close(id.clone()));

CloseGuard::drop from here is the only way to call DataInner::clear here.

This results in the DataInner not being freed (memory leak I believe), and the parent span not having it's ref_count decremented, leading to the parent span being stuck in limbo for eternity.

I ran into this when wrapping the registry type to have it behind an Arc to do some OTEL shenanigans with more than 32 tracing fields.