Update metrics.proto, and set metric descriptions in MetricsData messages

infra_libs/ts_mon/protos/metrics.proto

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 package ts_mon.proto;
 
 import "acquisition_network_device.proto";
 import "acquisition_task.proto";
@@ skipping 85 matching lines @@
     MEBIBYTES = 42;
     /** 1024^3 (1,073,741,824) bytes. */
     GIBIBYTES = 43;
 
     /** Extended Units */
     AMPS = 60;
     MILLIAMPS = 61;
     DEGREES_CELSIUS = 62;
   }
   optional Units units = 41;
+
+  optional string description = 43;
 }