Avoid encoding descriptor of empty family

src/encoding.rs

@@ -49,6 +49,17 @@ 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/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(