Update metric exporter to report lag as unit of time

[davidmparrott]

Exporter will now, additionally, report a metric for kafka consumer lag as a unit of time.

• Created additional function in collect to perform lag estimation
• Information about current produced offsets is added during consumer group metric collection
• Interpolation table is now separate file containing relevant struct and methods
• Prune added, which can be called at any time to iterate of the interpolation table and remove stale and overly large data
• Added ticker to perform pruning at a configurable interval

[davidmparrott]

closed in favor of #195 which includes mixin