monitoring: use HistogramVec.With() directly (#724)
monitoring: use HistogramVec.With() directly (#724)

Prometheus' HistogramVec no longer exports the underlying MetricVec,
so use the top-level With() method instead.
RJPercival/trillian,codingllama/trillian,google/trillian,AlCutter/trillian,gdbelvin/trillian,google/trillian,RJPercival/trillian,codingllama/trillian,daviddrysdale/trillian,gdbelvin/trillian,Martin2112/trillian,gdbelvin/trillian,Martin2112/trillian,Martin2112/trillian,AlCutter/trillian,daviddrysdale/trillian,daviddrysdale/trillian