Initializing metrics exporter

[LikeTheSalad]

We're currently initializing tracer and logger rexporters but there's no metrics exporter provided by default, nor there is an "exporter customizer" for metrics either.

[breedx-splk]

Do we want to even export metrics from mobile clients at all? We've discussed the cardinality and aggregation problems at length...

Even if we do choose to add this, I'm not sure that I would consider it a requirement for v1.0.0.

[LikeTheSalad]

We do not want to export metrics ourselves, though I don't think it's safe to assume that our preferences are shared by every user of this library.

Also, I believe one important aspect of a library is its behavior predictability and API consistency. Since we are already doing the exporter setup for 2 out of the 3 signal types of OTel, it seems strange that we just leave one out. Likewise for the APIs that allow to customize them.

The above is mainly the reason why I believe this should be a requirement for v1.0.0. Although I guess to be more specific, the requirement I think is more about making all signals behave the same way, though that doesn't mean that we must initialize a metric exporter, I guess another approach could be not initializing any exporter at all and provide setters for each instead.