Add Group struct.
Add Group struct.

A group can be, for example, "connections" which would contain gauges or
counters like "current" (read: current connections).

For now it has 3 methods:

    AddGauge(name, description)
    AddCounter(name, description)
    AddSummary(name, description)

lowstz/mongodb_exporter,dcu/mongodb_exporter,dcu/mongodb_exporter