fix(webapp): rotate ELU baseline so it reports per-interval, not cumulative

lastEventLoopUtilization was set once at init and never reassigned,
so every performance.eventLoopUtilization(current, last) diff was
computed against the process-start snapshot. The nodejs.event_loop.utilization
gauge was therefore a cumulative average over process lifetime rather
than a per-interval measurement.

Rotate the baseline immediately after computing the diff.

Author: ericallam

apps/webapp/app/v3/tracer.server.ts

@@ -717,6 +717,9 @@ function configureNodejsMetrics({ meter }: { meter: Meter }) {
       currentEventLoopUtilization,
       lastEventLoopUtilization
     );
+    // Rotate the baseline so the next collection reports per-interval
+    // utilization rather than the cumulative average from process start.
+    lastEventLoopUtilization = currentEventLoopUtilization;
 
     // diff.utilization is between 0 and 1 (fraction of time "active")
     const utilization = Number.isFinite(diff.utilization) ? diff.utilization : 0;