Expose the historical success probability calculation itself

Expose the historical success probability calculation itself

In 3f32f60ae7e75a4be96d3d5adc8d18b53445e5e5 we exposed the
historical success probability buckets directly, with a long method
doc explaining how to use it. While this is great for logging
exactly what the internal model thinks, its also helpful to let
users know what the internal model thinks the success probability
is directly, allowing them to compare route success probabilities.

Here we do so but only for the historical tracking buckets.