refactor(dcutr): reshape public API to follow naming guidelines (#3214)

With this patch, the naming of types follows the guidelines discussed in #2217.

misc/metrics/src/lib.rs

@@ -100,8 +100,8 @@ pub trait Recorder<Event> {
 }
 
 #[cfg(feature = "dcutr")]
-impl Recorder<libp2p_dcutr::behaviour::Event> for Metrics {
-    fn record(&self, event: &libp2p_dcutr::behaviour::Event) {
+impl Recorder<libp2p_dcutr::Event> for Metrics {
+    fn record(&self, event: &libp2p_dcutr::Event) {
         self.dcutr.record(event)
     }
 }