Library that exposes some metrics on replication state

[volkerstampa]

An eventuate application should be enabled to expose metrics about the replication state. In particular the following numbers should be exposed per replication endpoint and per log:

• current sequence number of the local log
• per remote log: sequence number in the remote log up to which events are replicated into the local log

Initial implementation is based on dropwizard's metrics library, but support for other metrics libraries can follow.