src/metrics: Implement info metrics

Open Metrics [`Info`] metric "to expose textual information which SHOULD
NOT change during process lifetime".

```
use open_metrics_client::metrics::info::Info;

let _info = Info::new(vec![("os", "GNU/linux")]);
```

README.md

@@ -28,8 +28,6 @@ behind the Open Metrics specification. Not being compliant with all requirements
 (`MUST` and `MUST NOT`) of the specification is considered a bug and likely to
 be fixed in the future. Contributions in all forms are most welcome.
 
-- Info metric.
-
 - State set metric.
 
 - Enforce "A Histogram MetricPoint MUST contain at least one bucket".

src/encoding/text.rs

@@ -29,6 +29,7 @@ use crate::metrics::exemplar::{CounterWithExemplar, Exemplar, HistogramWithExemp
 use crate::metrics::family::Family;
 use crate::metrics::gauge::{self, Gauge};
 use crate::metrics::histogram::Histogram;
+use crate::metrics::info::Info;
 use crate::metrics::{MetricType, TypedMetric};
 use crate::registry::{Registry, Unit};
 
@@ -167,6 +168,7 @@ impl Encode for MetricType {
             MetricType::Counter => "counter",
             MetricType::Gauge => "gauge",
             MetricType::Histogram => "histogram",
+            MetricType::Info => "info",
             MetricType::Unknown => "unknown",
         };
 
@@ -540,6 +542,28 @@ fn encode_histogram_with_maybe_exemplars<S: Encode>(
     Ok(())
 }
 
+/////////////////////////////////////////////////////////////////////////////////
+// Info
+
+impl<S> EncodeMetric for Info<S>
+where
+    S: Clone + std::hash::Hash + Eq + Encode,
+{
+    fn encode(&self, mut encoder: Encoder) -> Result<(), std::io::Error> {
+        encoder
+            .encode_suffix("info")?
+            .no_bucket()?
+            .encode_value(1u32)?
+            .no_exemplar()?;
+
+        Ok(())
+    }
+
+    fn metric_type(&self) -> MetricType {
+        Self::TYPE
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -639,6 +663,24 @@ mod tests {
         parse_with_python_client(String::from_utf8(encoded).unwrap());
     }
 
+    #[test]
+    fn encode_info() {
+        let mut registry = Registry::default();
+        let info = Info::new(vec![("os".to_string(), "GNU/linux".to_string())]);
+        registry.register("my_info_metric", "My info metric", info);
+
+        let mut encoded = Vec::new();
+        encode(&mut encoded, &registry).unwrap();
+
+        let expected = "# HELP my_info_metric My info metric.\n".to_owned()
+            + "# TYPE my_info_metric info\n"
+            + "my_info_metric_info 1\n"
+            + "# EOF\n";
+        assert_eq!(expected, String::from_utf8(encoded.clone()).unwrap());
+
+        parse_with_python_client(String::from_utf8(encoded).unwrap());
+    }
+
     #[test]
     fn encode_histogram() {
         let mut registry = Registry::default();

src/metrics.rs

@@ -5,6 +5,7 @@ pub mod exemplar;
 pub mod family;
 pub mod gauge;
 pub mod histogram;
+pub mod info;
 
 /// A metric that is aware of its Open Metrics metric type.
 pub trait TypedMetric {
@@ -16,11 +17,11 @@ pub enum MetricType {
     Counter,
     Gauge,
     Histogram,
+    Info,
     Unknown,
     // Not (yet) supported metric types.
     //
     // GaugeHistogram,
-    // Info,
     // StateSet,
     // Summary
 }

src/metrics/info.rs

@@ -0,0 +1,25 @@
+//! Module implementing an Open Metrics info metric.
+//!
+//! See [`Info`] for details.
+
+use crate::metrics::{MetricType, TypedMetric};
+
+/// Open Metrics [`Info`] metric "to expose textual information which SHOULD NOT
+/// change during process lifetime".
+///
+/// ```
+/// # use open_metrics_client::metrics::info::Info;
+///
+/// let _info = Info::new(vec![("os", "GNU/linux")]);
+/// ```
+pub struct Info<S>(S);
+
+impl<S> Info<S> {
+    pub fn new(label_set: S) -> Self {
+        Self(label_set)
+    }
+}
+
+impl<S> TypedMetric for Info<S> {
+    const TYPE: MetricType = MetricType::Info;
+}