Open Radar

 
We are seeing negative distance values from `CMPedometer` object sent to the updated handler of `-[CMPedometer startPedometerUpdatesFromDate:withHandler:]`. This seems to start at a negative value and then work its way forward as the users accrues distance:
```
2020-06-01 06:44:17.310	negative_pedometer_update	error		"{ "cumulative_distance": -1.4304558401927352 }"
2020-06-01 06:44:17.302	negative_pedometer_update	error		"{ "cumulative_distance": -3.0058954493142664 }"
2020-06-01 06:44:17.294	negative_pedometer_update	error		"{ "cumulative_distance": -4.581335058435798 }"
2020-06-01 06:44:17.286	negative_pedometer_update	error		"{ "cumulative_distance": -4.581335058435798 }"
2020-06-01 06:44:17.279	negative_pedometer_update	error		"{ "cumulative_distance": -4.581335058435798 }"
2020-06-01 06:44:17.272	negative_pedometer_update	error		"{ "cumulative_distance": -14.033972713630646 }"
2020-06-01 06:44:17.264	negative_pedometer_update	error		"{ "cumulative_distance": -14.033972713630646 }"
2020-06-01 06:44:17.257	negative_pedometer_update	error		"{ "cumulative_distance": -14.033972713630646 }"
2020-06-01 06:44:17.249	negative_pedometer_update	error		"{ "cumulative_distance": -14.033972713630646 }"
2020-06-01 06:44:17.240	negative_pedometer_update	error		"{ "cumulative_distance": -20.21515619382262 }"
2020-06-01 06:44:17.230	negative_pedometer_update	error		"{ "cumulative_distance": -31.243233458604664 }"
2020-06-01 06:44:17.220	negative_pedometer_update	error		"{ "cumulative_distance": -31.243233458604664 }"
2020-06-01 06:44:17.209	negative_pedometer_update	error		"{ "cumulative_distance": -31.243233458604664 }"
```
These are all from a single app user's session. The timestamps aren't super relevant as they're related to when these messages were transferred from watchOS to iOS. Extra events from a timeframe of `2020-05-28` to `2020-06-01` are included for reference. This illustrates the start from a negative value and work its way forward for multiple users.

The system does something like this when the issue seems to occur:

1. User says they want to start a workout within our application.
2. `-[HKHealthStore startWorkoutSession:]`
3. `-[CLLocationManager startUpdatingLocation]`
4. `-[CMPedometer startPedometerUpdatesFromDate:[NSDate date] withHandler:...]`
  - Started from `+[NSDate date]`, this handler dispatches to another object that moves the distance update to the main queue and then we log it on the main queue.
5. `-[CMMotionManager startAccelerometerUpdatesToQueue:withHandler:]`

Should we save the initial value from the `CMPedometer` update handler and then consider that our local `0` value? It seems like the API _should not_ return a negative value at any point especially for a series of values moving toward `0`.