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

@@ -32,10 +32,12 @@
 #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;
+extern const HAL_SITL& hal_sitl;
 
 /*
   parent class for all simulator types
@@ -470,13 +472,17 @@ void Aircraft::fill_fdm(struct sitl_fdm &fdm)
 // @Field: ASpdU: Achieved simulation speedup value
         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,
+            hal_sitl.get_uart_output_full_queue_count()
+        );
     }
 #endif
 }