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