Collect the hostname of the machine as a metric.

The hostname is collected during a build to determine where the user
is build from (from office workstation, cloudtop, etc...). This is a
useful metrics for RBE related builds.

Bug: b/169395325
Test: m nothing, ran printproto to verify that soong_metrics has the
      hostname.
Change-Id: I3e5d64f948ae43c5dda60de1d4a52b187e36dff8

ui/metrics/metrics_proto/metrics.proto

@@ -96,6 +96,9 @@ message MetricsBase {
   optional SoongBuildMetrics soong_build_metrics = 22;
 
   optional BuildConfig build_config = 23;
+
+  // The hostname of the machine.
+  optional string hostname = 24;
 }
 
 message BuildConfig {