use %r instead of %d to log PID change (#722)

turns out PID is set to None in the beginning,
leading to a crash when logging this message.

Author: beniwohli

elasticapm/base.py

@@ -207,7 +207,7 @@ def start_threads(self):
         with self._thread_starter_lock:
             current_pid = os.getpid()
             if self._pid != current_pid:
-                self.logger.debug("Detected PID change from %d to %d, starting threads", self._pid, current_pid)
+                self.logger.debug("Detected PID change from %r to %r, starting threads", self._pid, current_pid)
                 for manager_type, manager in self._thread_managers.items():
                     self.logger.debug("Starting %s thread", manager_type)
                     manager.start_thread()