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