should support sampling for timer metrics

[majewsky]

I'm sending statsd metrics from Openstack Swift to the statsd_exporter and seeing error messages like:

time="2017-01-06T10:18:08Z" level=error msg="Illegal sampling factor for non-counter metric on line swift.object-server.REPLICATE.timing:0.654935836792|ms|@0.1" source="exporter.go:390"

The statsd_exporter only allows sampling factors for counter metrics, but it should also do so for timer metrics. See for example https://github.com/etsy/statsd/blob/master/docs/metric_types.md#timing for upstream documentation that references sampling factors for timer metrics.

[majewsky]

This was apparently fixed in b5dfc9c.