Start collecting build configuration metrics.

This is an initial CL that collects the environment variables
USE_GOMA and USE_RBE. This is a needed metric to determine how
many users are currently using GOMA so we can help them migrate
to RBE.

Bug: b/163598127
Test: m nothing and checked soong_metrics
Change-Id: I39536eabad01aafbb3bd7182515d72230b36f658