[Darwin] Always capture last value for metric end event (project-chip#39733)

- When collecting metrics for end event, make sure to always capture the
  value allowing overwrite of the value if it has already been logged
  previously. This is to allow last writer always wins.

Co-authored-by: Andrei Litvin <[email protected]>

Author: anush-apple

src/darwin/Framework/CHIP/MTRMetricsCollector.mm

@@ -240,8 +240,8 @@ - (void)handleMetricEvent:(MetricEvent)event
         }
     }
 
-    // Add to the collection only if it does not exist as yet.
-    if (![_metricsDataCollection valueForKey:metricsKey]) {
+    // Add to the collection only if it does not exist as yet or pick latest value for instant event
+    if (![_metricsDataCollection valueForKey:metricsKey] || event.type() == MetricEvent::Type::kInstantEvent) {
         [_metricsDataCollection setValue:data forKey:metricsKey];
     }
 }