Add PullingGauge (#405)

Signed-off-by: Jan Berktold <jberktold@roblox.com>

Signed-off-by: Jan Berktold <jberktold@roblox.com>

src/lib.rs

@@ -158,6 +158,7 @@ mod errors;
 mod gauge;
 mod histogram;
 mod metrics;
+mod pulling_gauge;
 #[cfg(feature = "push")]
 mod push;
 mod registry;
@@ -219,6 +220,7 @@ pub use self::histogram::DEFAULT_BUCKETS;
 pub use self::histogram::{exponential_buckets, linear_buckets};
 pub use self::histogram::{Histogram, HistogramOpts, HistogramTimer, HistogramVec};
 pub use self::metrics::Opts;
+pub use self::pulling_gauge::PullingGauge;
 #[cfg(feature = "push")]
 pub use self::push::{
     hostname_grouping_key, push_add_collector, push_add_metrics, push_collector, push_metrics,

src/pulling_gauge.rs

@@ -0,0 +1,101 @@
+use std::{collections::HashMap, fmt, sync::Arc};
+
+use crate::{
+    core::Collector,
+    proto::{Gauge, Metric, MetricFamily, MetricType},
+};
+use protobuf::RepeatedField;
+
+/// A [Gauge] that returns the value from a provided function on every collect run.
+///
+/// This metric is the equivalant of Go's
+/// <https://pkg.go.dev/github.com/prometheus/client_golang@v1.11.0/prometheus#GaugeFunc>
+///
+/// # Examples
+/// ```
+/// # use prometheus::{Registry, PullingGauge};
+/// # // We are stubbing out std::thread::available_parallelism since it's not available in the
+/// # // oldest Rust version that we support.
+/// # fn available_parallelism() -> f64 { 0.0 }
+///
+/// let registry = Registry::new();
+/// let gauge = PullingGauge::new(
+///     "available_parallelism",
+///     "The available parallelism, usually the numbers of logical cores.",
+///     Box::new(|| available_parallelism())
+/// ).unwrap();
+/// registry.register(Box::new(gauge));
+/// ```
+#[derive(Clone)]
+pub struct PullingGauge {
+    desc: crate::core::Desc,
+    value: Arc<Box<dyn Fn() -> f64 + Send + Sync>>,
+}
+
+impl fmt::Debug for PullingGauge {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("PullingGauge")
+            .field("desc", &self.desc)
+            .field("value", &"<opaque>")
+            .finish()
+    }
+}
+
+impl PullingGauge {
+    /// Create a new [`PullingGauge`].
+    pub fn new<S1: Into<String>, S2: Into<String>>(
+        name: S1,
+        help: S2,
+        value: Box<dyn Fn() -> f64 + Send + Sync>,
+    ) -> crate::Result<Self> {
+        Ok(PullingGauge {
+            value: Arc::new(value),
+            desc: crate::core::Desc::new(name.into(), help.into(), Vec::new(), HashMap::new())?,
+        })
+    }
+
+    fn metric(&self) -> Metric {
+        let mut gauge = Gauge::default();
+        let getter = &self.value;
+        gauge.set_value(getter());
+
+        let mut metric = Metric::default();
+        metric.set_gauge(gauge);
+
+        metric
+    }
+}
+
+impl Collector for PullingGauge {
+    fn desc(&self) -> Vec<&crate::core::Desc> {
+        vec![&self.desc]
+    }
+
+    fn collect(&self) -> Vec<crate::proto::MetricFamily> {
+        let mut m = MetricFamily::default();
+        m.set_name(self.desc.fq_name.clone());
+        m.set_help(self.desc.help.clone());
+        m.set_field_type(MetricType::GAUGE);
+        m.set_metric(RepeatedField::from_vec(vec![self.metric()]));
+        vec![m]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::metrics::Collector;
+
+    #[test]
+    fn test_pulling_gauge() {
+        const VALUE: f64 = 10.0;
+
+        let gauge =
+            PullingGauge::new("test_gauge", "Purely for testing", Box::new(|| VALUE)).unwrap();
+
+        let metrics = gauge.collect();
+        assert_eq!(metrics.len(), 1);
+
+        assert_eq!(VALUE, metrics[0].get_metric()[0].get_gauge().get_value());
+    }
+}