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
diff --git a/ui/metrics/metrics.go b/ui/metrics/metrics.go
index 12f74dd..f5552a3 100644
--- a/ui/metrics/metrics.go
+++ b/ui/metrics/metrics.go
@@ -66,6 +66,10 @@
 	}
 }
 
+func (m *Metrics) BuildConfig(b *soong_metrics_proto.BuildConfig) {
+	m.metrics.BuildConfig = b
+}
+
 func (m *Metrics) SetMetadataMetrics(metadata map[string]string) {
 	for k, v := range metadata {
 		switch k {