add stack info to "client set more than once" log message (#1158)

This should allow us to find the call chain that triggers
such a case

Author: beniwohli

elasticapm/base.py

@@ -631,5 +631,5 @@ def set_client(client):
     global CLIENT_SINGLETON
     if CLIENT_SINGLETON:
         logger = get_logger("elasticapm")
-        logger.debug("Client object is being set more than once")
+        logger.debug("Client object is being set more than once", stack_info=True)
     CLIENT_SINGLETON = client