feat: allow Arc<String> on EncodeLabelSet

CHANGELOG.md

@@ -21,8 +21,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Support `i32`/`f32` for `Gauge` and `u32`/`f32` for `Counter`/`CounterWithExemplar`.
   See [PR 173] and [PR 216].
 
+- Supoort `Arc<String>` for `EncodeLabelValue`.
+  See [PR 217].
+
 [PR 173]: https://github.com/prometheus/client_rust/pull/173
 [PR 216]: https://github.com/prometheus/client_rust/pull/216
+[PR 217]: https://github.com/prometheus/client_rust/pull/217
 
 ## [0.22.3]
 

derive-encode/tests/lib.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 use prometheus_client::encoding::text::encode;
 use prometheus_client::encoding::{EncodeLabelSet, EncodeLabelValue};
 use prometheus_client::metrics::counter::Counter;
@@ -137,6 +139,36 @@ fn remap_keyword_identifiers() {
     assert_eq!(expected, buffer);
 }
 
+#[test]
+fn arc_string() {
+    #[derive(EncodeLabelSet, Hash, Clone, Eq, PartialEq, Debug)]
+    struct Labels {
+        client_id: Arc<String>,
+    }
+
+    let mut registry = Registry::default();
+    let family = Family::<Labels, Counter>::default();
+    registry.register("my_counter", "This is my counter", family.clone());
+
+    // Record a single HTTP GET request.
+    let client_id = Arc::new("client_id".to_string());
+    family
+        .get_or_create(&Labels {
+            client_id: client_id.clone(),
+        })
+        .inc();
+
+    // Encode all metrics in the registry in the text format.
+    let mut buffer = String::new();
+    encode(&mut buffer, &registry).unwrap();
+
+    let expected = "# HELP my_counter This is my counter.\n".to_owned()
+        + "# TYPE my_counter counter\n"
+        + "my_counter_total{client_id=\"client_id\"} 1\n"
+        + "# EOF\n";
+    assert_eq!(expected, buffer);
+}
+
 #[test]
 fn flatten() {
     #[derive(EncodeLabelSet, Hash, Clone, Eq, PartialEq, Debug)]

src/encoding.rs

@@ -468,12 +468,19 @@ impl EncodeLabelValue for &str {
         Ok(())
     }
 }
+
 impl EncodeLabelValue for String {
     fn encode(&self, encoder: &mut LabelValueEncoder) -> Result<(), std::fmt::Error> {
         EncodeLabelValue::encode(&self.as_str(), encoder)
     }
 }
 
+impl EncodeLabelValue for &String {
+    fn encode(&self, encoder: &mut LabelValueEncoder) -> Result<(), std::fmt::Error> {
+        EncodeLabelValue::encode(&self.as_str(), encoder)
+    }
+}
+
 impl<'a> EncodeLabelValue for Cow<'a, str> {
     fn encode(&self, encoder: &mut LabelValueEncoder) -> Result<(), std::fmt::Error> {
         EncodeLabelValue::encode(&self.as_ref(), encoder)