test: add test for using empty struct as histogram family labels

src/encoding/text.rs

@@ -569,6 +569,7 @@ mod tests {
     use crate::metrics::{counter::Counter, exemplar::CounterWithExemplar};
     use pyo3::{prelude::*, types::PyModule};
     use std::borrow::Cow;
+    use std::fmt::Error;
 
     #[test]
     fn encode_counter() {
@@ -764,6 +765,33 @@ mod tests {
         parse_with_python_client(encoded);
     }
 
+    #[test]
+    fn encode_histogram_family_with_empty_struct_family_labels() {
+        let mut registry = Registry::default();
+        let family =
+            Family::new_with_constructor(|| Histogram::new(exponential_buckets(1.0, 2.0, 10)));
+        registry.register("my_histogram", "My histogram", family.clone());
+
+        #[derive(Eq, PartialEq, Hash, Debug, Clone)]
+        struct EmptyLabels {}
+
+        impl EncodeLabelSet for EmptyLabels {
+            fn encode(&self, _encoder: crate::encoding::LabelSetEncoder) -> Result<(), Error> {
+                Ok(())
+            }
+        }
+
+        family
+            .get_or_create(&EmptyLabels {})
+            .observe(1.0);
+
+        let mut encoded = String::new();
+
+        encode(&mut encoded, &registry).unwrap();
+
+        parse_with_python_client(encoded);
+    }
+
     #[test]
     fn encode_histogram_with_exemplars() {
         let mut registry = Registry::default();