metrics: event-loop lag excludes the sampling interval (an idle loop read ~20 ms before)

ddiff --git a/metrics.js b/metrics.js
iindex 4e9e99e..51e311e 100644
---- a/metrics.js
++++ b/metrics.js
@@@ -286,11 +286,16 @@ function processMetrics(registry, { eventLoopResolutionMs = 20 } = {}) {
          cpu.set((c.user + c.system) / 1e6);
          uptime.set(Math.round(process.uptime()));
          if (loop && lag) {
--            // The histogram is in nanoseconds; with no samples yet it reports NaN/huge values.
++            // The histogram is in nanoseconds and each sample includes the sampling interval itself
++            // (an idle loop reads ~resolution), so the interval is subtracted: what is left is lag.
++            // With no samples yet it reports NaN/huge values, so nothing is claimed.
              const ok = loop.count > 0 || loop.max > 0;
--            lag.set({ stat: 'mean' }, ok ? loop.mean / 1e9 : 0);
--            lag.set({ stat: 'p99' }, ok ? loop.percentile(99) / 1e9 : 0);
--            lag.set({ stat: 'max' }, ok ? loop.max / 1e9 : 0);
++            const lagOf = (ns) => Math.max(0, ns / 1e9 - eventLoopResolutionMs / 1000);
++            if (ok) {
++                lag.set({ stat: 'mean' }, lagOf(loop.mean));
++                lag.set({ stat: 'p99' }, lagOf(loop.percentile(99)));
++                lag.set({ stat: 'max' }, lagOf(loop.max));
++            }
              loop.reset();
          }
      });