Enable multiple aggregation types in Value Metric

This change allows clients to specify what aggregation types should be
used on a per value field basis. Previously, only one aggregation type
was allowed and was applied to every value field.

Test: atest statsd_test
Bug: 324462987
Change-Id: Ie5dd122c7b8343cc4af7691f2b3a7edf586e4d3c
14 files changed