Event metrics

Added a query for getting data points for event metrics + cumulative aggregation fold for combining event metric points.
Yeah, but the only time range you care about is still the one from the use site (Borel), say you have:

```
event: small             medium
time : 0           1       2              3 (now)       
borel:          start                     end
```

Do you still need query time ``0-3``? Can you just query ``1-3`` and still know which instance it was before it changed to medium at ``t=2``?