Capture the build command that executed the build system.

We need to capture the user build command to understand what additional
arguments is passed in to Soong. This data is useful to know which
targets are being executed and have statistics on top build targets
are specified.

Bug: b/169453712
Test: ran "m blueprint_tools", "m nothing", "lunch 1" and checked
      in soong_metrics proto.
Change-Id: I7ab94dcf037211f1abffe6afd012b03ffbf99756
diff --git a/ui/metrics/metrics_proto/metrics.proto b/ui/metrics/metrics_proto/metrics.proto
index 44f8859..8d3933f 100644
--- a/ui/metrics/metrics_proto/metrics.proto
+++ b/ui/metrics/metrics_proto/metrics.proto
@@ -102,6 +102,9 @@
 
   // The system resource information such as total physical memory.
   optional SystemResourceInfo system_resource_info = 25;
+
+  // The build command that the user entered to the build system.
+  optional string build_command = 26;
 }
 
 message BuildConfig {