MongoDB: Adding replication lag metric

[victorgp]

To get replication lag data i needed to obtain it from a different command "replSetGetStatus".

The code was meant to work only with a single command "serverStatus", to allow getting data from different places/commands, i changed the structure a little bit
I created a MongoStatus type that holds any other type from any other command, in this case, it has a ServerStatus object (the one that existed before) and a ReplSetStatus object, a new one that i created to store the data from "replSetGetStatus".

Using this approach, adding more metrics that come from different commands is a matter of adding the attribute to the MongoStatus type.

I tested this with a replicated setup and with a non replicated, for the non replicated it won't do anything.