SITL: log number of times sim paused on serial0 buffer

SITL pauses the simulation if we do not have a minimum amount of space in its out queue.

Log the number of times we do this.

libraries/SITL/SIM_Aircraft.cpp

@@ -32,11 +32,17 @@
 #include <AP_JSON/AP_JSON.h>
 #include <AP_Filesystem/AP_Filesystem.h>
 #include <AP_AHRS/AP_AHRS.h>
+#include <AP_HAL_SITL/HAL_SITL_Class.h>
 
 using namespace SITL;
 
 extern const AP_HAL::HAL& hal;
 
+// the SITL HAL can add information about pausing the simulation and its effect on the UART.  Not present when we're compiling for simulation-on-hardware
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+extern const HAL_SITL& hal_sitl;
+#endif
+
 /*
   parent class for all simulator types
  */
@@ -452,6 +458,14 @@ void Aircraft::fill_fdm(struct sitl_fdm &fdm)
     }
 
 #if HAL_LOGGING_ENABLED
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+    // the SITL HAL can add information about pausing the simulation
+    // and its effect on the UART.  Not present when we're compiling
+    // for simulation-on-hardware
+    const uint32_t full_count = hal_sitl.get_uart_output_full_queue_count();
+#else
+    const uint32_t full_count = 0;
+#endif
     // for EKF comparison log relhome pos and velocity at loop rate
     static uint16_t last_ticks;
     uint16_t ticks = AP::scheduler().ticks();
@@ -468,15 +482,20 @@ void Aircraft::fill_fdm(struct sitl_fdm &fdm)
 // @Field: VD: Velocity down
 // @Field: As: Airspeed
 // @Field: ASpdU: Achieved simulation speedup value
+// @Field: UFC: Number of times simulation paused for serial0 output
         Vector3d pos = get_position_relhome();
         Vector3f vel = get_velocity_ef();
-        AP::logger().WriteStreaming("SIM2", "TimeUS,PN,PE,PD,VN,VE,VD,As,ASpdU",
-                                    "Qdddfffff",
-                                    AP_HAL::micros64(),
-                                    pos.x, pos.y, pos.z,
-                                    vel.x, vel.y, vel.z,
-                                    airspeed_pitot,
-                                    achieved_rate_hz/rate_hz);
+        AP::logger().WriteStreaming(
+            "SIM2",
+            "TimeUS,PN,PE,PD,VN,VE,VD,As,ASpdU,UFC",
+            "QdddfffffI",
+            AP_HAL::micros64(),
+            pos.x, pos.y, pos.z,
+            vel.x, vel.y, vel.z,
+            airspeed_pitot,
+            achieved_rate_hz/rate_hz,
+            full_count
+        );
     }
 #endif
 }