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This is a reland of 2fe77415

Original change's description:
> [Structured metrics] Add metric collection and upload logic.
> 
> This is the final CL for the core of the structured metrics system. It
> ties together the KeyData class (for managing keys and computing hashes)
> with the metrics provider, and implements logic to:
> 
>  1. Hash and store incoming structured metrics events.
>  2. Provide stored metrics events for upload.
> 
> It also adds integration tests for the system, checking that an event
> created through the public API is correctly prepared for upload.
> 
> Other misc changes:
> 
>  - Some minor refactoring of event_base.{h,cc} and equivalent changes
>    to events_template.py, to fix some bad style.
>  - Added a missing RemoveObserver call to the metrics provider dtor.
> 
> Bug: 1016655
> Change-Id: I3daae2ba7927742d26db8d5750538584bf13b94f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975386
> Commit-Queue: Tony Yeoman <tby@chromium.org>
> Reviewed-by: Andrew Moylan <amoylan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#735748}

Bug: 1016655
Change-Id: I83e69de7a9b865d35036b714f068cfda3c423b76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026913Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736590}