ValueMetric supports multiple aggregation types

1. Add support for MIN, MAX, AVG
2. ValueMetric also allow floats now, in addition to long data type.
AnomalyDetection still takes long only. I am not sure if it makes
sense to do anomaly on AVG. I will leave that for later.
3. ValueMetric supports sliced condition change for pushed events.
I don't think it makes sense for pulled events to have sliced condition
changes so leave it for now.

Test: unit test
Change-Id: I8bc510d98ea9b8a6eb16d04ff99dce6b574249cd