Add input_download timings to run-bubblewrap / adjust download timing start

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

If not required, please explain in brief why not.

Description

The bubblewrap runner doesn't populate the input_download request metadata, which makes the bgd-browser page look pretty empty. This PR fixes that and has buildbox-run-bubblewrap correctly set the input_download and output_upload timings.

As part of adding that I noticed that buildbox-run-userchroot starts the download timing only around the stageTree call. This seems sensible, but misses all the downloads necessary for any tree merging which might be required due to chrootRootDigest. The fetchChrootIntoLocalCas call in particular can add a bunch of time if the digest isn't already in the local cache.

Changes proposed in this merge request:

• Add missing RequestMetadata timings to buildbox-run-bubblewrap
• Adjust the start of input_download to include time fetching chrootRootDigest / doing tree merging in buildbox-run-userchroot