Add RBE metrics dump in Soong UI.

From aosp/1329396, the RBE metrics protobuf file is part of the
metrics uploading process. The RBE metrics protobuf file is
generated by running the bootstrap shutdown command. A new function
named DumpRBEMetrics was written in order to generate the RBE metrics
protobuf file before sending to the uploading process.

Bug: b/140638454
Test: * Unit test cases
      * Ran RBE build on my local host and verified the
        metrics protobuf file is created.
      * Ran non-RBE build after RBE build and verified that
        the previous metrics protobuf file was deleted.

Change-Id: I4b8068905cb67c4b8c2d94793917b98974fed707

ui/metrics/metrics.go

@@ -25,12 +25,13 @@ import (
 )
 
 const (
-	RunSetupTool = "setup"
-	RunKati      = "kati"
-	RunSoong     = "soong"
-	PrimaryNinja = "ninja"
-	TestRun      = "test"
-	Total        = "total"
+	PrimaryNinja    = "ninja"
+	RunKati         = "kati"
+	RunSetupTool    = "setup"
+	RunShutdownTool = "shutdown"
+	RunSoong        = "soong"
+	TestRun         = "test"
+	Total           = "total"
 )
 
 type Metrics struct {