Remove dimension fields in GaugeMetric output

GaugeMetric output is atom format and can contain all fields including
those already included in dimensions.
This is not necessary and duplicate.
And, if we do set dimensions, we can use this to reduce data size
similar to repeated fields, where same dimension value only appear once.

Bug: 113061955
Test: unit test
Change-Id: I299bd1cb1b9b90ea7426ef182df78d2ffc091910