Fix metrics "connection_client" and "connecting_client" underreportin…

…g due to multiple client actors of a connection starting on the same instance.

Tagging the metrics by the client actor number is not possible
without significant change because the client actor props factory
does not know about client actor numbers.

Signed-off-by: Yufei Cai <yufei.cai@bosch.io>

connectivity/service/src/main/java/org/eclipse/ditto/connectivity/service/messaging/BaseClientActor.java

@@ -190,7 +190,9 @@ public abstract class BaseClientActor extends AbstractFSMWithStash<BaseClientSta
     private final ActorRef connectionActor;
     private final ActorSelection commandForwarderActorSelection;
     private final Gauge clientGauge;
+    private boolean clientGaugeFlag = false; // set if clientGauge is incremented
     private final Gauge clientConnectingGauge;
+    private boolean clientConnectingGaugeFlag = false; // set if clientConnectingGauge is incremented
     private final ReconnectTimeoutStrategy reconnectTimeoutStrategy;
     private final SupervisorStrategy supervisorStrategy;
     private final ConnectionPubSub connectionPubSub;
@@ -369,8 +371,12 @@ private boolean shouldAnyTargetSendConnectionAnnouncements() {
     @Override
     public void postStop() {
         cancelOnStopTasks.forEach(Cancellable::cancel);
-        clientGauge.reset();
-        clientConnectingGauge.reset();
+        if (clientGaugeFlag) {
+            clientGauge.decrement();
+        }
+        if (clientConnectingGaugeFlag) {
+            clientConnectingGauge.decrement();
+        }
         stopChildActor(tunnelActor);
         logger.debug("Stopped client with id - <{}>", getDefaultClientId());
         try {
@@ -613,19 +619,23 @@ protected final List<Source> getSourcesOrEmptyList() {
     private void onTransition(final BaseClientState from, final BaseClientState to) {
         logger.debug("Transition: {} -> {}", from, to);
         if (to == CONNECTED) {
-            clientGauge.set(1L);
+            clientGauge.increment();
+            clientGaugeFlag = true;
             reconnectTimeoutStrategy.reset();
             publishConnectionOpenedAnnouncement();
         }
         if (to == DISCONNECTED) {
-            clientGauge.reset();
+            clientGauge.decrement();
+            clientGaugeFlag = false;
         }
         if (to == CONNECTING) {
-            clientConnectingGauge.set(1L);
+            clientConnectingGauge.increment();
+            clientConnectingGaugeFlag = true;
         }
         // do not use else if since we might use goTo(CONNECTING) if in CONNECTING state. This will cause another onTransition.
         if (from == CONNECTING) {
-            clientConnectingGauge.reset();
+            clientConnectingGauge.decrement();
+            clientConnectingGaugeFlag = false;
         }
         // cancel our own state timeout if target state is stable
         if (to == CONNECTED || to == DISCONNECTED || to == INITIALIZED) {