Bug 2018675 - Do not use a callback interface for MetricsHandler

[freshstrangemusic]

Using a callback interface forces the type to come in as Box<T> instead of Arc<T> as a regular interface. We're storing it as an Arc<Box<T>> because we need it to be Send, so using a plain interface allows us to drop the Box and store a Arc<T> directly, preventing an additional layer of indirection.

Because the MetricsHandler is in an Arc, we can keep an Arc<TestMetrics> around and use it directly instead of dowcasting the reference inside Arc<dyn MetricsHandler> and so we can remove the NimbusClient::get_metrics_handler() test method.

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