Upload build metrics after a build is completed.

Soong now supports the ability to upload metrics to another location
by setting the ANDROID_ENABLE_METRICS_UPLOAD to an uploader that
accepts the upload.proto proto buffer message. When the environment
variable is set, a set of build metrics files (soong_metrics,
rbe_metrics.pb and build_error) is uploaded.

Bug: 140638454
Test: * Wrote unit test cases
      * Setup the uploader, built a succcessful and failed aosp_arm-eng
        target and monitor the uploading of the metrics.
Change-Id: I76a65739c557dc90345e098ca03119a950ece2d2
diff --git a/ui/build/config.go b/ui/build/config.go
index d66a86c..49f506e 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -961,3 +961,14 @@
 func (c *configImpl) IsPdkBuild() bool {
 	return c.pdkBuild
 }
+
+func (c *configImpl) BuildDateTime() string {
+	return c.buildDateTime
+}
+
+func (c *configImpl) MetricsUploaderApp() string {
+	if p, ok := c.environ.Get("ANDROID_ENABLE_METRICS_UPLOAD"); ok {
+		return p
+	}
+	return ""
+}