This actually matches what is already [documented in the header](https://github.com/10gen/mongo/blob/master/src/mongo/otel/metrics/metrics_initialization.h#L24). The reason this is needed is that a metric may be registered during shutdown, so the provider is nullptr, but the code assumes the provider is always nonnull (e.g. [here](https://github.com/10gen/mongo/blob/32af7511ad7d933b46e21ee3569af3f73f40ee61/src/mongo/otel/metrics/metrics_service.h#L650)). My guess is that the author assumed setting the provider to `nullptr`or to `{}` (which results in `nullptr` becuse it creates a default-constructed `shared_ptr`) would set the provider to noop.

Additionally, removed some useless comments from the non-otel implementations.