Expose KVStore file system latency histograms via Prometheus

KV team has a way to publish their latency histograms to Prometheus. We should do the same for Couchstore / Magma's file system histogram (ReadTimeHisto, WriteTimeHisto).

Relevant code to look at:

Magma already adds its histograms in MagmaKVStore::addTimingStats which gets called for cbstats.

We have to do similar wiring for Prometheus. KV's PrometheusStatsCollector supports taking in HdrHistogram (the superclass of all histograms) - see PrometheusStatCollector::addStat.

KV currently pushes its own histograms in the following code path:
EventuallyPersistentEngine::get_prometheus_stats
EventuallyPersistentEngine::doMetricGroupHigh
EventuallyPersistentEngine::doTimingStats.