Add the possibility of using Exometer for the metrics instead of Folsom

I don't know whether you find this useful or not ... I added the possibility of calling the Exometer API for the metrics storage instead of the Folsom one.

There are two configuration parameters `{metrics_module, callback_module}`  and ` {metrics_api,  exometer | folsom}`. In this way it is possible to configure which API will be called and also to provide a custom callback module if wanted.