message: Explain metrics timestamp subtraction in runner constructor
diff: @@ -764,6 +764,8 @@ impl Runner<InitializerReactor> {
  reactor,
  event_count: 0,
  metrics: RunnerMetrics::new(&registry)?,
  + // Calculate the `last_metrics` timestamp to be exactly one delay in the past. This will
  + // cause the runner to collect metrics at the first opportunity.
  last_metrics: now.checked_sub(event_metrics_min_delay).unwrap_or(now),
  event_metrics_min_delay,
  event_metrics_threshold: 1000,
  