Add simple getter for polling_period in Logger

Change-Id: I5d15413371e3b9d31bc236afe7463bb72593847e
diff --git a/aos/events/logging/log_writer.h b/aos/events/logging/log_writer.h
index 992633a..4f85eb6 100644
--- a/aos/events/logging/log_writer.h
+++ b/aos/events/logging/log_writer.h
@@ -62,6 +62,7 @@
   void set_polling_period(std::chrono::nanoseconds polling_period) {
     polling_period_ = polling_period;
   }
+  std::chrono::nanoseconds polling_period() const { return polling_period_; }
 
   std::string_view log_start_uuid() const { return log_start_uuid_; }
   UUID logger_instance_uuid() const { return logger_instance_uuid_; }