Add method to merge a LabelSet into a Metric with collision avoidance.

Colliding labels can happen e.g. when an exporter job is scraped and already
includes "job" labels for its samples in /metrics. In this case, a
collisionPrefix of "exporter_" is added to the colliding target labels, but the
specifics (the collision prefix) are managed by Prometheus, not the client
library.
Merge pull request #27 from prometheus/refactor/metric-label-merging

Add method to merge a LabelSet into a Metric with collision avoidance.