Allow the customisation of the base namespace of the statsd namespace

As it is all stats are aggregated in `mongobetween.` namespace.
If you have a set of pools, all of the once are shown in the current namespace.

We should be able to add a new parameter to modify the namespace, so we can have more granular things that include host name as well.
Maybe expose a parameter or something.

Start by locating where mongobetween builds and emits the `mongobetween.` statsd namespace, then inspect how configuration parameters are defined. Decide how a configurable base namespace should interact with pool and host names, and verify that separate pools produce distinct metric namespaces.

Written by the indexing model from the issue text.