feat: allow to encode a registry in openmetrics without EOF

src/registry.rs

@@ -81,24 +81,40 @@ impl Registry {
         registry.register_all(metrics_group_set)
     }
 
-    fn encode_inner(&self, writer: &mut impl Write) -> fmt::Result {
+    /// Encodes all metrics in the OpenMetrics text format.
+    ///
+    /// This does not write the terminal `# EOF\n` string to `writer`.
+    /// You can use [`encode_openmetrics_eof`] to do that.
+    pub fn encode_openmetrics_to_writer(&self, writer: &mut impl Write) -> fmt::Result {
         for group in &self.metrics {
             group.encode_openmetrics(writer, self.prefix.as_deref(), &self.labels)?;
         }
 
         for sub in self.sub_registries.iter() {
-            sub.encode_inner(writer)?;
+            sub.encode_openmetrics_to_writer(writer)?;
         }
         Ok(())
     }
 }
 
+/// Writes `# EOF\n` to `writer`.
+///
+/// This is the expected last characters of an OpenMetrics string.
+pub fn encode_openmetrics_eof(writer: &mut impl Write) -> fmt::Result {
+    write_eof(writer)
+}
+
 /// Helper trait to abstract over different ways to access metrics.
 pub trait MetricsSource: Send + 'static {
     /// Encodes all metrics into a string in the OpenMetrics text format.
+    ///
+    /// This is expected to also write the terminal `# EOF\n` string expected
+    /// by the OpenMetrics format.
     fn encode_openmetrics(&self, writer: &mut impl std::fmt::Write) -> Result<(), Error>;
 
     /// Encodes the metrics in the OpenMetrics text format into a newly allocated string.
+    ///
+    /// See also [`Self::encode_openmetrics`].
     fn encode_openmetrics_to_string(&self) -> Result<String, Error> {
         let mut s = String::new();
         self.encode_openmetrics(&mut s)?;
@@ -108,7 +124,7 @@ pub trait MetricsSource: Send + 'static {
 
 impl MetricsSource for Registry {
     fn encode_openmetrics(&self, writer: &mut impl std::fmt::Write) -> Result<(), Error> {
-        self.encode_inner(writer)?;
+        self.encode_openmetrics_to_writer(writer)?;
         write_eof(writer)?;
         Ok(())
     }

src/static_core.rs

@@ -35,7 +35,7 @@ use std::sync::OnceLock;
 
 use erased_set::ErasedSyncSet;
 
-use crate::{Error, MetricsGroup, NoMetricsSnafu, Registry};
+use crate::{Error, MetricsGroup, MetricsSource, NoMetricsSnafu, Registry};
 
 #[cfg(not(feature = "metrics"))]
 type Registry = ();
@@ -46,7 +46,7 @@ type Registry = ();
 #[derive(Clone, Copy, Debug)]
 pub struct GlobalRegistry;
 
-impl crate::MetricsSource for GlobalRegistry {
+impl MetricsSource for GlobalRegistry {
     fn encode_openmetrics(&self, writer: &mut impl std::fmt::Write) -> Result<(), Error> {
         let core = crate::static_core::Core::get().ok_or(NoMetricsSnafu.build())?;
         core.registry.encode_openmetrics(writer)
@@ -86,7 +86,7 @@ impl Core {
             #[cfg(feature = "metrics")]
             registry,
         })
-        .map_err(|_| std::io::Error::new(std::io::ErrorKind::Other, "already set"))
+        .map_err(|_| std::io::Error::other("already set"))
     }
 
     /// Returns a reference to the core metrics.