src/metrics: Introduce const types

Signed-off-by: Max Inden <mail@max-inden.de>

src/metrics/counter.rs

@@ -187,6 +187,38 @@ where
     }
 }
 
+/// As [`Counter`] but constant, that is can not change once created.
+///
+/// Needed for advanced use-cases, e.g. in combination with [`Collector`](crate::registry::Collector).
+#[derive(Debug, Default)]
+pub struct ConstCounter<N = u64> {
+    value: N,
+}
+
+impl<N> ConstCounter<N> {
+    /// Creates a new [`ConstCounter`].
+    pub fn new(value: N) -> Self {
+        Self { value }
+    }
+}
+
+impl<N> TypedMetric for ConstCounter<N> {
+    const TYPE: MetricType = MetricType::Counter;
+}
+
+impl<N> EncodeMetric for ConstCounter<N>
+where
+    N: crate::encoding::EncodeCounterValue,
+{
+    fn encode(&self, mut encoder: MetricEncoder) -> Result<(), std::fmt::Error> {
+        encoder.encode_counter::<(), _, u64>(&self.value, None)
+    }
+
+    fn metric_type(&self) -> MetricType {
+        Self::TYPE
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/metrics/family.rs

@@ -326,6 +326,27 @@ where
     }
 }
 
+// TODO: When we split the dispatched type for the standard registered metrics
+// and the collector metrics and only require Sync for the former, we can use a
+// RefCell here.
+impl<S: EncodeLabelSet, M: EncodeMetric + TypedMetric, T: Iterator<Item = (S, M)>> EncodeMetric
+    for parking_lot::Mutex<T>
+{
+    fn encode(&self, mut encoder: MetricEncoder<'_, '_>) -> Result<(), std::fmt::Error> {
+        let mut iter = self.lock();
+
+        while let Some((label_set, m)) = iter.next() {
+            let encoder = encoder.encode_family(&label_set)?;
+            m.encode(encoder)?;
+        }
+        Ok(())
+    }
+
+    fn metric_type(&self) -> MetricType {
+        M::TYPE
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/metrics/gauge.rs

@@ -253,6 +253,37 @@ where
     }
 }
 
+/// As [`Gauge`] but constant, that is can not change once created.
+///
+/// Needed for advanced use-cases, e.g. in combination with [`Collector`](crate::registry::Collector).
+#[derive(Debug, Default)]
+pub struct ConstGauge<N = i64> {
+    value: N,
+}
+
+impl<N> ConstGauge<N> {
+    /// Creates a new [`ConstGauge`].
+    pub fn new(value: N) -> Self {
+        Self { value }
+    }
+}
+
+impl<N> TypedMetric for ConstGauge<N> {
+    const TYPE: MetricType = MetricType::Gauge;
+}
+
+impl<N> EncodeMetric for ConstGauge<N>
+where
+    N: EncodeGaugeValue,
+{
+    fn encode(&self, mut encoder: MetricEncoder) -> Result<(), std::fmt::Error> {
+        encoder.encode_gauge(&self.value)
+    }
+    fn metric_type(&self) -> MetricType {
+        Self::TYPE
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;