message: [metrics] add easy timers for histograms
  Update service_metrics to add timers to use with histograms.
  The histogram is updated when the timer is dropped from the scope -- useful for duration logging.
diff: use prometheus::{
  core::{Collector, Desc},
  proto::MetricFamily,
  - Histogram, HistogramOpts, HistogramVec, IntCounter, IntCounterVec, IntGauge, IntGaugeVec, Opts,
  + Histogram, HistogramOpts, HistogramTimer, HistogramVec, IntCounter, IntCounterVec, IntGauge,
  + IntGaugeVec, Opts,
  };
  #[derive(Clone)]
  @@ -116,6 +117,10 @@ impl OpMetrics {
  pub fn observe(&self, op: &str, v: f64) {
  self.histograms.with_label_values(&[op]).observe(v);
  }
  +
  + pub fn timer(&self, op: &str) -> HistogramTimer {
  + self.histograms.with_label_values(&[op]).start_timer()
  + }
  }
  impl Collector for OpMetrics {
  