sensor.proto: result for set_sampling_rate() rpc

sensor.proto

@@ -35,7 +35,14 @@ message DataChunk {
     repeated Temperature data = 3;
 }
 
-message SetSamplingPeriodResult {}
+enum SetSamplingPeriodResultEnum {
+    OK = 0;
+    OutOfRange = 1;
+}
+
+message SetSamplingPeriodResult {
+    SetSamplingPeriodResultEnum result = 1;
+}
 
 service Sensor {
     rpc get_status (StatusRequest) returns (SensorStatus) {}