Implement allocation-friendly method to get user count in SimpUserRegistry

SPR-14930

spring-messaging/src/main/java/org/springframework/messaging/simp/user/MultiServerUserRegistry.java

@@ -135,6 +135,16 @@ public class MultiServerUserRegistry implements SimpUserRegistry, SmartApplicati
 		return result;
 	}
 
+	@Override
+	public int getUserCount() {
+		int userCount = 0;
+		for (UserRegistrySnapshot registry : this.remoteRegistries.values()) {
+			userCount += registry.getUserMap().size();
+		}
+		userCount += this.localRegistry.getUserCount();
+		return userCount;
+	}
+
 	@Override
 	public Set<SimpSubscription> findSubscriptions(SimpSubscriptionMatcher matcher) {
 		Set<SimpSubscription> result = new HashSet<>();

spring-messaging/src/main/java/org/springframework/messaging/simp/user/SimpUserRegistry.java

@@ -40,6 +40,13 @@ public interface SimpUserRegistry {
 	 */
 	Set<SimpUser> getUsers();
 
+	/**
+	 * Return the count of all connected users.
+	 * @return the connected user count.
+	 * @since 4.3.5
+	 */
+	int getUserCount();
+
 	/**
 	 * Find subscriptions with the given matcher.
 	 * @param matcher the matcher to use