feat(metrics): Implement exemplar metrics

foundations-metrics-registry/src/encode_metric.rs

@@ -6,5 +6,8 @@ use crate::proto::MetricFamily;
 /// metric or series that fails, so an empty `Vec` is a valid result.
 pub trait EncodeMetric: Send + Sync + 'static {
     /// Encodes this metric into zero or more [`MetricFamily`] messages.
+    ///
+    /// Every returned [`MetricFamily`] must set `name` to a complete, non-empty
+    /// producer-level name.
     fn encode(&self) -> Vec<MetricFamily>;
 }

foundations-metrics-registry/src/metadata.rs

@@ -4,7 +4,7 @@
 /// [`register`](crate::register). Build it from [`default`](Self::default) plus
 /// the setters, since downstream crates can't use a struct literal.
 #[non_exhaustive]
-#[derive(Clone, Default)]
+#[derive(Clone, Debug, Default)]
 pub struct RegistrationMetadata {
     /// Whether the metric is exported only when optional metrics are requested.
     pub optional: bool,

foundations-metrics-registry/src/proto/mod.rs

@@ -10,7 +10,8 @@
 mod model;
 
 pub use model::{
-    Bucket, BucketSpan, Counter, Gauge, Histogram, LabelPair, Metric, MetricFamily, MetricType,
+    Bucket, BucketSpan, Counter, Exemplar, Gauge, Histogram, LabelPair, Metric, MetricFamily,
+    MetricType, Quantile, Summary,
 };
 
 #[cfg(test)]

foundations-metrics/Cargo.toml

@@ -15,6 +15,8 @@ prometheus-client = { version = "0.25.0", features = [
 serde = { workspace = true, features = ["derive"] }
 ryu = "1.0.23"
 parking_lot = { workspace = true }
+prost = { workspace = true }
+prost-types = { workspace = true }
 
 [lints]
 workspace = true