AC_AttitudeControl: record sample time in attitude controller

libraries/AC_AttitudeControl/AC_AttitudeControl.h

@@ -73,9 +73,14 @@ class AC_AttitudeControl {
 
     // set_dt / get_dt - dt is the time since the last time the attitude controllers were updated
     // _dt should be set based on the time of the last IMU read used by these controllers
+    // sample_time_us is the time of the last IMU read used by these controllers
     // the attitude controller should run updates for active controllers on each loop to ensure normal operation
-    void set_dt(float dt) { _dt = dt; }
+    void set_dt(float dt, uint64_t sample_time_us) {
+        _dt = dt;
+        _sample_time_us = sample_time_us;
+    }
     float get_dt() const { return _dt; }
+    uint64_t get_sample_time_us() const { return _sample_time_us; }
 
     // pid accessors
     AC_P& get_angle_roll_p() { return _p_angle_roll; }
@@ -489,6 +494,8 @@ class AC_AttitudeControl {
 
     // Intersampling period in seconds
     float               _dt;
+    // scheduler sample time in microseconds (could be different to rate loop)
+    uint64_t            _sample_time_us;
 
     // This represents a 321-intrinsic rotation in NED frame to the target (setpoint)
     // attitude used in the attitude controller, in radians.