fix(subscriber): ignore metadata that is not a span or event (#554)

As described in #553, there are parts of code that expects tracing
`Metadata` to be of type `EVENT` or `SPAN`.

This PR ensure that any tracing metadata with a different type will be
ignored instead of panicking when running in debug mode when converting
from `tracing_core::Metadata`:
https://github.com/tokio-rs/console/blob/60bcf87537d414b21efbd84159207f9ecda5e914/console-api/src/common.rs#L48

console-subscriber/src/lib.rs

@@ -529,6 +529,10 @@ where
     S: Subscriber + for<'a> LookupSpan<'a>,
 {
     fn register_callsite(&self, meta: &'static Metadata<'static>) -> subscriber::Interest {
+        if !meta.is_span() && !meta.is_event() {
+            return subscriber::Interest::never();
+        }
+
         let dropped = match (meta.name(), meta.target()) {
             ("runtime.spawn", _) | ("task", "tokio::task") => {
                 self.spawn_callsites.insert(meta);