Make subscription latency estimation in MTRDevice a bit less noisy.

Averages out the new latency with (lower weighted) previous ones.

Author: bzbarsky-apple

src/darwin/Framework/CHIP/MTRDevice.mm

@@ -306,6 +306,13 @@ - (BOOL)isEqual:(id)object
 // happen more often than once every 10 minutes.
 #define MTRDEVICE_MIN_RESUBSCRIBE_DUE_TO_READ_INTERVAL_SECONDS (10 * 60)
 
+// Weight of new data in determining subscription latencies.  To avoid random
+// outliers causing too much noise in the value, treat an existing value (if
+// any) as having 2/3 weight and the new value as having 1/3 weight.  These
+// weights are subject to change, if it's determined that different ones give
+// better behavior.
+#define MTRDEVICE_SUBSCRIPTION_LATENCY_NEW_VALUE_WEIGHT (1.0 / 3.0)
+
 @interface MTRDevice ()
 @property (nonatomic, readonly) os_unfair_lock lock; // protects the caches and device state
 // protects against concurrent time updates by guarding timeUpdateScheduled flag which manages time updates scheduling,
@@ -1012,7 +1019,12 @@ - (void)_handleSubscriptionEstablished
         // We want time interval from initialSubscribeStart to now, not the other
         // way around.
         NSTimeInterval subscriptionLatency = -[initialSubscribeStart timeIntervalSinceNow];
-        _estimatedSubscriptionLatency = @(subscriptionLatency);
+        if (_estimatedSubscriptionLatency == nil) {
+            _estimatedSubscriptionLatency = @(subscriptionLatency);
+        } else {
+            NSTimeInterval newSubscriptionLatencyEstimate = MTRDEVICE_SUBSCRIPTION_LATENCY_NEW_VALUE_WEIGHT * subscriptionLatency + (1 - MTRDEVICE_SUBSCRIPTION_LATENCY_NEW_VALUE_WEIGHT) * _estimatedSubscriptionLatency.doubleValue;
+            _estimatedSubscriptionLatency = @(newSubscriptionLatencyEstimate);
+        }
         [self _storePersistedDeviceData];
     }