swarm-review-classifier: test that Otel::record exports the run's cost

Nothing exercised the real record path: the acp tests use a mock sink and
the cost tests only covered amount(). Split the counter construction and
the add out of Otel::record so a test can run them against an in-memory
exporter, and assert the counter name, the amount and the four labels.

Dev-dependency: opentelemetry_sdk's `testing` feature (same crate; adds
tokio to its lockfile entry).

swarm-review-classifier/Cargo.toml

@@ -33,6 +33,8 @@ tracing.workspace = true
 
 [dev-dependencies]
 tempfile = "3"
+# `InMemoryMetricExporter` for the cost counter test.
+opentelemetry_sdk = { workspace = true, features = ["testing"] }
 
 [lints]
 workspace = true

swarm-review-classifier/src/cost.rs

@@ -66,18 +66,31 @@ impl CostSink for Otel {
         let Some((_, counter)) = COUNTER.get_or_init(build) else {
             return;
         };
-        counter.add(
-            cost.amount,
-            &[
-                KeyValue::new("repo", cost.repo.clone()),
-                KeyValue::new("pr", i64::try_from(cost.pr).unwrap_or(i64::MAX)),
-                KeyValue::new("model", cost.model.clone()),
-                KeyValue::new("currency", CURRENCY),
-            ],
-        );
+        add(counter, cost);
     }
 }
 
+/// The counter on `provider`.
+fn cost_counter(provider: &SdkMeterProvider) -> Counter<f64> {
+    provider
+        .meter("hyperhive.review_classifier")
+        .f64_counter("hyperhive.review_classifier.cost.usage")
+        .build()
+}
+
+/// Adds `cost` to `counter` with the run's labels.
+fn add(counter: &Counter<f64>, cost: &Cost) {
+    counter.add(
+        cost.amount,
+        &[
+            KeyValue::new("repo", cost.repo.clone()),
+            KeyValue::new("pr", i64::try_from(cost.pr).unwrap_or(i64::MAX)),
+            KeyValue::new("model", cost.model.clone()),
+            KeyValue::new("currency", CURRENCY),
+        ],
+    );
+}
+
 /// The provider, kept alive with the counter since it exports only while it
 /// lives, or `None` when no collector is configured or the exporter failed.
 fn build() -> Option<(SdkMeterProvider, Counter<f64>)> {
@@ -101,16 +114,18 @@ fn build() -> Option<(SdkMeterProvider, Counter<f64>)> {
         .with_interval(INTERVAL)
         .build();
     let provider = SdkMeterProvider::builder().with_reader(reader).build();
-    let counter = provider
-        .meter("hyperhive.review_classifier")
-        .f64_counter("hyperhive.review_classifier.cost.usage")
-        .build();
+    let counter = cost_counter(&provider);
     Some((provider, counter))
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
+    use opentelemetry::Value;
+    use opentelemetry_sdk::metrics::InMemoryMetricExporter;
+    use opentelemetry_sdk::metrics::data::{
+        AggregatedMetrics, MetricData, ResourceMetrics, ScopeMetrics,
+    };
 
     fn usage(input: u64, output: u64, cache_read: u64, cache_write: u64) -> TokenUsage {
         let mut usage = TokenUsage::default();
@@ -147,4 +162,56 @@ mod tests {
         let cost = amount(&prices, &usage(0, 0, 500_000, 500_000));
         assert!((cost - 2.0).abs() < 1e-12, "{cost}");
     }
+
+    #[test]
+    fn a_record_adds_the_amount_under_the_counter_name_and_the_run_labels() {
+        let exporter = InMemoryMetricExporter::default();
+        let reader = PeriodicReader::builder(exporter.clone()).build();
+        let provider = SdkMeterProvider::builder().with_reader(reader).build();
+        let counter = cost_counter(&provider);
+        let prices = Prices {
+            input: 3.0,
+            output: 15.0,
+            cache_read: None,
+            cache_write: None,
+        };
+        let cost = Cost {
+            repo: "hyperhive/hyperhive".to_owned(),
+            pr: 5214,
+            model: "some-model".to_owned(),
+            amount: amount(&prices, &usage(1_000_000, 100_000, 0, 0)),
+        };
+        add(&counter, &cost);
+        provider.force_flush().expect("flush");
+
+        let metrics = exporter.get_finished_metrics().expect("metrics");
+        let metric = metrics
+            .iter()
+            .flat_map(ResourceMetrics::scope_metrics)
+            .flat_map(ScopeMetrics::metrics)
+            .find(|m| m.name() == "hyperhive.review_classifier.cost.usage")
+            .expect("the cost counter was exported");
+        let AggregatedMetrics::F64(MetricData::Sum(sum)) = metric.data() else {
+            panic!("not an f64 sum: {:?}", metric.data());
+        };
+        let points: Vec<_> = sum.data_points().collect();
+        assert_eq!(points.len(), 1);
+        let point = points[0];
+        assert!((point.value() - 4.5).abs() < 1e-12, "{}", point.value());
+        let labels: Vec<(String, Value)> = point
+            .attributes()
+            .map(|kv| (kv.key.to_string(), kv.value.clone()))
+            .collect();
+        let label = |key: &str| {
+            labels
+                .iter()
+                .find(|(k, _)| k == key)
+                .map(|(_, v)| v.clone())
+        };
+        assert_eq!(label("repo"), Some(Value::from("hyperhive/hyperhive")));
+        assert_eq!(label("pr"), Some(Value::from(5214_i64)));
+        assert_eq!(label("model"), Some(Value::from("some-model")));
+        assert_eq!(label("currency"), Some(Value::from("USD")));
+        assert_eq!(labels.len(), 4);
+    }
 }