fix(encoding): do not encode descriptor of empty family

CHANGELOG.md

@@ -12,7 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   fewer `as` casts in their implementation. This caught an issue where
   `EncodeGaugeValue` would not error when encoding some `u64`s that don't fit
   in a `i64`. See [PR 281].
+- Filter out empty metric families, to match the go client. See [PR 279].
 
+[PR 279]: https://github.com/prometheus/client_rust/pull/279
 [PR 281]: https://github.com/prometheus/client_rust/pull/281
 
 ## [0.24.0]

src/encoding.rs

@@ -50,6 +50,17 @@ pub trait EncodeMetric {
     // One can not use [`TypedMetric`] directly, as associated constants are not
     // object safe and thus can not be used with dynamic dispatching.
     fn metric_type(&self) -> MetricType;
+
+    /// Check if the metric is empty.
+    ///
+    /// An empty metric is a metric that has no data to encode, and thus should not have any
+    /// descriptor in the final output.
+    ///
+    /// By default, this returns `false`, ensuring the metric and its description is always
+    /// encoded.
+    fn is_empty(&self) -> bool {
+        false
+    }
 }
 
 impl EncodeMetric for Box<dyn EncodeMetric> {

src/encoding/text.rs

@@ -1278,4 +1278,41 @@ def parse(input):
                 .unwrap();
         })
     }
+
+    #[test]
+    fn encode_omit_empty() {
+        let mut registry = Registry::default();
+        let counter1: Family<Vec<(&'static str, &'static str)>, Counter> = Default::default();
+        let counter2: Family<Vec<(&'static str, &'static str)>, Counter> = Default::default();
+        let counter3: Family<Vec<(&'static str, &'static str)>, Counter> = Default::default();
+
+        registry.register("counter1", "First counter", counter1.clone());
+        registry.register("counter2", "Second counter", counter2.clone());
+        registry.register("counter3", "Third counter", counter3.clone());
+
+        counter1.get_or_create(&vec![("label", "value")]).inc();
+
+        let mut encoded = String::new();
+        encode(&mut encoded, &registry).unwrap();
+
+        let expected = "# HELP counter1 First counter.\n".to_owned()
+            + "# TYPE counter1 counter\n"
+            + "counter1_total{label=\"value\"} 1\n"
+            + "# EOF\n";
+        assert_eq!(expected, encoded);
+
+        counter2.get_or_create(&vec![("label", "value")]).inc();
+
+        let mut encoded = String::new();
+        encode(&mut encoded, &registry).unwrap();
+
+        let expected = "# HELP counter1 First counter.\n".to_owned()
+            + "# TYPE counter1 counter\n"
+            + "counter1_total{label=\"value\"} 1\n"
+            + "# HELP counter2 Second counter.\n"
+            + "# TYPE counter2 counter\n"
+            + "counter2_total{label=\"value\"} 1\n"
+            + "# EOF\n";
+        assert_eq!(expected, encoded);
+    }
 }

src/metrics/family.rs

@@ -377,6 +377,10 @@ where
     fn metric_type(&self) -> MetricType {
         M::TYPE
     }
+
+    fn is_empty(&self) -> bool {
+        self.metrics.read().is_empty()
+    }
 }
 
 #[cfg(test)]

src/registry.rs

@@ -287,7 +287,7 @@ impl Registry {
     }
 
     pub(crate) fn encode(&self, encoder: &mut DescriptorEncoder) -> Result<(), std::fmt::Error> {
-        for (descriptor, metric) in self.metrics.iter() {
+        for (descriptor, metric) in self.metrics.iter().filter(|(_, m)| !m.is_empty()) {
             let mut descriptor_encoder =
                 encoder.with_prefix_and_labels(self.prefix.as_ref(), &self.labels);
             let metric_encoder = descriptor_encoder.encode_descriptor(