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).
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