Bugfix: Impl TypedMetric for Exemplar types

This allows my families-exemplars example to compile. The `latency` metric in my example can have a family label and an exemplar label:

```
latency_sum{result="success"} 0.001345422
latency_count{result="success"} 1
latency_bucket{result="success",le="1.0"} 1 # {trace_id="3a2f90c9f80b894f"} 0.001345422
```

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   set from a family. See [PR 85].
 - Added a `clear` method to `Family` to allow the removal of all label sets
   from a family. See [PR 85].
+- Impl `TypedMetric` for `CounterWithExemplar` and `HistogramWithExemplar`, so that they can be used with `Family`. See [PR 96].
 
 ### Changed
 

examples/families-exemplars.rs

@@ -1,5 +1,5 @@
 use prometheus_client::{
-    encoding::text::SendSyncEncodeMetric,
+    encoding::text::encode,
     metrics::{exemplar::HistogramWithExemplars, family::Family, histogram::exponential_buckets},
     registry::Registry,
 };
@@ -16,13 +16,24 @@ fn main() {
             HistogramWithExemplars::new(exponential_buckets(1.0, 2.0, 10))
         });
 
-    // This line doesn't compile:
-    //
-    // the trait `TypedMetric` is not implemented for `HistogramWithExemplar<TraceLabel>`
-    let b: Box<dyn SendSyncEncodeMetric> = Box::new(latency);
-
     // Register metrics.
-    registry.register("latency", "help text goes here", b);
+    registry.register("latency", "help text goes here", Box::new(latency.clone()));
+
+    latency
+        .get_or_create(&ResultLabel {
+            result: "success".to_owned(),
+        })
+        .observe(
+            0.001345422,
+            Some(TraceLabel {
+                trace_id: "3a2f90c9f80b894f".to_owned(),
+            }),
+        );
+
+    let mut buf = Vec::new();
+    encode(&mut buf, &registry).unwrap();
+    let line = std::str::from_utf8(buf.as_slice()).unwrap().to_string();
+    println!("{line}");
 }
 
 #[derive(Clone, Hash, PartialEq, Eq, Encode, Debug, Default)]
@@ -32,5 +43,5 @@ pub struct ResultLabel {
 
 #[derive(Clone, Hash, PartialEq, Eq, Encode, Debug, Default)]
 pub struct TraceLabel {
-    trace_id: String,
+    pub trace_id: String,
 }

src/metrics/exemplar.rs

@@ -4,6 +4,7 @@
 
 use super::counter::{self, Counter};
 use super::histogram::Histogram;
+use super::{MetricType, TypedMetric};
 use parking_lot::{MappedRwLockReadGuard, RwLock, RwLockReadGuard};
 use std::collections::HashMap;
 #[cfg(any(target_arch = "mips", target_arch = "powerpc"))]
@@ -37,6 +38,10 @@ pub struct CounterWithExemplar<S, N = u64, A = AtomicU64> {
     pub(crate) inner: Arc<RwLock<CounterWithExemplarInner<S, N, A>>>,
 }
 
+impl<S> TypedMetric for CounterWithExemplar<S> {
+    const TYPE: MetricType = MetricType::Counter;
+}
+
 /// Open Metrics [`Counter`] with an [`Exemplar`] to both measure discrete
 /// events and track references to data outside of the metric set.
 #[cfg(any(target_arch = "mips", target_arch = "powerpc"))]
@@ -128,6 +133,10 @@ pub struct HistogramWithExemplars<S> {
     pub(crate) inner: Arc<RwLock<HistogramWithExemplarsInner<S>>>,
 }
 
+impl<S> TypedMetric for HistogramWithExemplars<S> {
+    const TYPE: MetricType = MetricType::Histogram;
+}
+
 impl<S> Clone for HistogramWithExemplars<S> {
     fn clone(&self) -> Self {
         Self {