Address review: align interface param names with implementation

Rename `openConnections`/`connectionsCounter` to `statsName`/`number`
to match DefaultTransportService and avoid misleading future readers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

common/transport/transport-api/src/main/java/org/thingsboard/server/common/transport/TransportService.java

@@ -161,5 +161,5 @@ public interface TransportService {
 
     boolean hasSession(SessionInfoProto sessionInfo);
 
-    void createGaugeStats(String openConnections, AtomicInteger connectionsCounter, String... tags);
+    void createGaugeStats(String statsName, AtomicInteger number, String... tags);
 }