Xing have you begun this?  I was about to start tonight.  I'll hold off if you're working on it though.

I have started.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/385565129bf692d60470af4e1dc960247c6fdf21

commit 385565129bf692d60470af4e1dc960247c6fdf21
Author: Xing Liu <xingliu@chromium.org>
Date: Tue Jul 18 03:10:56 2017

Add metrics for download service.

This CL implements metrics for download service.

Bug:  736090 
Change-Id: I7da7c02f2af9c0071b7c21519cc93e2fcc486687
Reviewed-on: https://chromium-review.googlesource.com/551108
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487367}
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/content/internal/download_driver_impl.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/controller.h
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/controller_impl.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/controller_impl_unittest.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/download_service_impl.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/download_service_impl_unittest.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/entry.h
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/file_monitor_impl.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/model_impl.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/model_impl_unittest.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/proto_conversions.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/stats.cc
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/internal/stats.h
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/public/client.h
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/components/download/public/download_params.h
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/385565129bf692d60470af4e1dc960247c6fdf21/tools/metrics/histograms/histograms.xml