Pausing Metrics Collection

pauseMetrics() and resumeMetrics() to dynamically stop and resume metrics collection

This feature allows users to dynamically toggle metrics collection off.

pauseMetrics() will cause the agent to no longer send metrics via zmq or StatsD.

Internally metrics are collected and recalculated every duration of NSOLID_INTERVAL (default: 3 seconds).

When sending metrics is paused the metrics will still be collected and recalculated internally every NSOLID_INTERVAL. This way when the metrics are again sent they aren't skewed for the duration they were paused.