Construct metrics registry with default() when name_in_metrics is empty

[djhworld]

This adds a clause to use Registry::default() if name_in_metrics is empty

At the moment using the with_prefix method, the metrics that get rendered ends up looking like this: _my_metric because prefix is Option https://github.com/prometheus/client_rust/blob/master/src/registry.rs#L62

Using Registry::default() will set prefix to None so that metrics get rendered without the leading underscore.