Add pico/imu reset counters to IMU status

Attempt to track when the pico and/or IMU reset based on when the data
counter and pico timestamps jump unexpectedly.

Notes:
* Not actually tested on robot.
* It would be good to get these values exposed on a webpage.

Change-Id: I5ac89ce9493017bc58a741a302b5cdc856fbdcf6
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/frc971/imu_reader/imu_watcher.h b/frc971/imu_reader/imu_watcher.h
index fe1f77d..2b32371 100644
--- a/frc971/imu_reader/imu_watcher.h
+++ b/frc971/imu_reader/imu_watcher.h
@@ -81,6 +81,8 @@
 
   // Last observed pico measurement. Used to track IMU staleness.
   std::optional<aos::monotonic_clock::time_point> last_pico_timestamp_;
+  // Time at which we received the last imu message on the pi's clock.
+  std::optional<aos::monotonic_clock::time_point> last_imu_message_;
   // Estimate of the drift between the pi and pico clocks. See
   // pico_offset_error() for definition.
   aos::monotonic_clock::duration pico_offset_error_;