Add Constellation support for express & slow P3A metrics

[DJAndries]

Resolves brave/brave-browser#33217

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run lint, npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Use MITM proxy when using browser.

0. Start with fresh profile.
1. Ensure JSON metrics are sent as usual to p3a-json.brave.com. Ensure that the cadence field in the payload contains the correct value. If the metric is in the logs object (in local state), ensure the cadence is typical. If the metric is in logs_slow, ensure the cadence is slow. If the metric is in logs_express, ensure the cadence is express. Ensure that each metric sent to the server is marked as "sent" in the local state.
2. Ensure requests are being made to star-randsrv.bsg.brave.com to prepare Constellation metrics. Ensure that the URL path is /instances/<cadence type>/randomness. When one request is sent for a particular cadence, check to see that metrics are being marked as "sent" as they are being sent to the server, by checking the correct "logs" object in local state. For the typical cadence, keep an eye on logs_constellation_prep. For the express cadence, keep an eye on logs_constellation_prep_express. For the slow cadence, keep an eye on logs_constellation_prep_slow.
3. Ensure final measurements are being sent to collector.bsg.brave.com. The URL path should be /<cadence type>. Before metrics are sent to this server, they should appear under constellation_logs, constellation_logs_express and constellation_logs_slow after being prepared (from the requests being made in step 2). As soon as the metrics are sent to the collector, the metric should disappear in the relevant local state object.

[bridiver]

brave_domains url changes look ok

[bridiver]

this can be done with a chromium_src override for MigrateObsoleteLocalStatePrefs

[kylehickinson]

Since this is how we've managed the overrides for the other 3 methods can we keep this for now and then I'll open a new PR removing the patch entirely after this merges?

[bridiver]

I think it wasn't possible in the other cases because the method name wasn't unique in the file. This is a very simple change to a chromium_src override

[kylehickinson]

Yeah I just double checked, can only be done for MigrateObsoleteLocalStatePrefs and MigrateObsoleteBrowserStatePrefs. I'll update both

[bridiver]

there's probably still another way to make it work without patches in the other cases, but for now I just want to make sure we're not adding new patches

[kylehickinson]

For the time being, replaced the migrate methods over in this PR: 8a9fb74

[kylehickinson]

iOS++ tested locally with the BraveP3AConstellation feature flag enabled

[iefremov]

this is actually an anti-pattern, see code samples in https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/c++/checks.md

[kylehickinson]

If uplifted please also uplift #21000