Bug 2072330 - record metrics on large downloads

[bendk]

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• Quality: This PR builds and tests run cleanly
  • Note:
    • For changes that need extra cross-platform testing, consider adding [ci full] to the PR title.
    • If this pull request includes a breaking change, consider cutting a new release after merging.
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry in CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due diligence applied in selecting them.

[mhammond]

this seems slightly worrying if we later use viaduct for urls that may have pii, and it also seems like we are kinda abusing error reporting to record what it really just telemetry, but maybe we can suck this up if we remove it once we figure out what it going on? The other option would I guess be standard glean telemetry and we record the size of all successful downloads, but that's going to be more work and as of today probably needs to be in swift/kotlin.

@moztcampbell wdyt?

[bendk]

Good point about the PII. We could limit this to remote settings downloads. I believe those are the most likely culprits and the shouldn't have any PII in them.

I actually think reporting these as errors is reasonable since the current viaduct API isn't really intended to be used for huge downloads. I would also be for recording the size of all downloads using some glean metric (maybe a histogram?), but I see that as complimentary.