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).

Cargo.lock

@@ -3341,6 +3341,7 @@ dependencies = [
  "portable-atomic",
  "rand 0.9.5",
  "thiserror 2.0.18",
+ "tokio",
 ]
 
 [[package]]