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_proto/metrics.proto b/ui/metrics/metrics_proto/metrics.proto
index 50810eb..6559ba3 100644
--- a/ui/metrics/metrics_proto/metrics.proto
+++ b/ui/metrics/metrics_proto/metrics.proto
@@ -94,6 +94,14 @@
   optional PerfInfo total = 21;
 
   optional SoongBuildMetrics soong_build_metrics = 22;
+
+  optional BuildConfig build_config = 23;
+}
+
+message BuildConfig {
+  optional bool use_goma = 1;
+
+  optional bool use_rbe = 2;
 }
 
 message PerfInfo {
@@ -159,4 +167,4 @@
 
   // The approximate maximum size of the heap in soong_build in bytes.
   optional uint64 max_heap_size = 5;
-}
\ No newline at end of file
+}