feat(http/prom): add an error labeler

this commit introduces a `LabelError<E>` type, which provides a
`StreamLabel` implementation that maps boxed errors to labels.

this is generic across `E`-typed labels that can be constructed `From` a
reference to a boxed `linkerd_error::Error`.

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

Author: cratelyn

linkerd/http/prom/src/stream_label.rs

@@ -3,6 +3,7 @@
 use linkerd_error::Error;
 use prometheus_client::encoding::EncodeLabelSet;
 
+pub mod error;
 pub mod status;
 
 /// A strategy for labeling request/responses streams for status and duration

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

@@ -0,0 +1,35 @@
+//! [`StreamLabel`] implementation for labeling errors.
+
+use super::StreamLabel;
+use linkerd_error::Error;
+
+/// A [`StreamLabel`] implementation that maps boxed errors to labels.
+#[derive(Clone, Debug, Default)]
+pub struct LabelError<E> {
+    error: Option<E>,
+}
+
+// === impl LabelError ===
+
+impl<E> StreamLabel for LabelError<E>
+where
+    E: for<'a> From<&'a Error>,
+    E: Clone + Send + 'static,
+{
+    type DurationLabels = ();
+    type StatusLabels = Option<E>;
+
+    fn init_response<B>(&mut self, _: &http::Response<B>) {}
+
+    fn end_response(&mut self, res: Result<Option<&http::HeaderMap>, &Error>) {
+        let Err(err) = res else { return };
+        let labels = E::from(err);
+        self.error = Some(labels);
+    }
+
+    fn status_labels(&self) -> Self::StatusLabels {
+        self.error.clone()
+    }
+
+    fn duration_labels(&self) -> Self::DurationLabels {}
+}