[DeviceSync v2] Add metrics to device manager

These metrics track
  - DeviceSync v2 invocation reasons,
  - DeviceSync v2 results,
  - async ClientAppMetadata fetch success rates,
  - async ClientAppMetadata fetch execution times for tuning timeout.

Bug: 951969, 933656, 936273
Change-Id: I1cf1e1fb2ab2fd4c07ee740783daecb2611ac86f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776217
Commit-Queue: Josh Nohle <nohle@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735683}