Add metrics for cancelled loads

[stasimus]

Raised in #369 (comment).

Since #369 a load started by getOrUpdate can be cancelled, but CacheMetered only reports load(time, success) after value.attempt completes, so a cancelled load leaves no trace in cache_load_result or cache_load_time. The existing labels are result=success|failure.

Report cancelled loads as well, as result=cancelled, with the time spent until cancellation.