Make mapper cache respect metric type

[andy-paine]

• Statsd allows users to provide a metric with the same name but
  differing types (counter, gauge, timer)
• The exporter allows this by letting users specify a
  "match_metric_type" in the mapping config
• However the mapper cache does not look at the metric type so it would
  return a MetricMapperCacheResult for the type of the first metric with
  that name that the exporter saw
• Add MetricType to the signature for the cache and format the metric
  name with the type to provide unique keys for a metric with the same
  name but differing type

Fixes #228

[SpencerMalone]

To me this looks good, do you have thoughts on adding a test? If not, I'm good with this going in, and I can open a followup PR for a test to try and ensure this edge case isn't missed in any future work.

[andy-paine]

My Golang isn't all that great - I tried writing some tests but given the lack of tests in general in this repo I didn't really have a pattern to follow and don't feel in a position to be defining patterns for a whole repo. I figured I would raise this now and potentially look to add some tests later.

[matthiasr]

ok, I'll merge it then and wait for the test from @SpencerMalone