feat(http/prom): introduce MkWithLabels::new() (#4418)

#4251 introduced a suite of MkStreamLabel
components that can be used to inspect and label traffic. MkWithLabels
offers a way to construct this type externally, now that we will have
use for it in the inbound proxy.

Signed-off-by: katelyn martin <kate@buoyant.io>

Author: cratelyn

linkerd/http/prom/src/stream_label/with.rs

@@ -21,6 +21,12 @@ pub struct WithLabels<L> {
 
 // === impl MkWithLabels ===
 
+impl<L> MkWithLabels<L> {
+    pub fn new(labels: L) -> Self {
+        Self { labels }
+    }
+}
+
 impl<L> MkStreamLabel for MkWithLabels<L>
 where
     L: Clone + Send + 'static,