CallMetricRecorder is not test-friendly

Currently only `InternalCallMetricRecorder` can create `CallMetricRecorder`s. If an service implementation records metrics and wants to test that behavior, it cannot do so unless it uses the internal class.

Start by reading the CallMetricRecorder and InternalCallMetricRecorder classes to understand how recorders are currently created and which visibility or construction constraint prevents service implementations from testing metric behavior. Confirm the intended public usage and identify existing tests around these classes; done means a service implementation can create or use a recorder in a test without relying on the internal class.

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