multi-value aggregation in ValueMetric

Allow aggregation on multiple fields, instead of one at a time.
All these fields should use the same aggregation time, use_diff,
direction, etc.
The config reuses value_field but allows multiple fields to be
specified.
The order they are specified determines the "index" of a value in the
output.

Bug: 119217634
Test: unit test
Change-Id: I38b1465d13723a897b30ee0b4f868498f60ad4db