feat: add MetaMetrics custom flush vars and log

[NicolasMassart]

Description

• add optional custom flush vars and log in MetaMetrics Segment client config
• update example .js.env

Note

No unit test change as it only affect the log and it's dev feature.
The behaviour of metrics is unchanged.
If you feel tests should be added, please suggest how.

Related issues

Fixes #8772

Manual testing steps

0. With dev environment configured to use Segment (configure SEGMENT_WRITE_KEY and SEGMENT_PROXY_URL, see contrib doc)
1. Set env vars in .js.env:

export SEGMENT_FLUSH_INTERVAL=2
export SEGMENT_FLUSH_EVENT_LIMIT=4

2. run app in as a local dev build
3. check logs for:

LOG  [MetaMask DEBUG]: MetaMetrics client configured with: {
  "writeKey": "...",
  "proxy": "...",
  "debug": true,
  "anonymousId": "...",
  "flushInterval": "2",
  "flushAt": "4"
}

4. Navigate in the app to settings and notice the following two lines in the logs, very close to each other as interval is 2s:

INFO  TRACK event saved {"event": "Navigation Drawer", "properties": {"action": "Navigation Drawer", "name": "Settings"}, "type": "track"}
...
INFO  Sent 1 events 

Screenshots/Recordings

Before

NA

After

NA

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained what problem this PR is solving and how it is solved.
• I've linked related issues
• I've included manual testing steps
• I've included screenshots/recordings if applicable
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.
• I’ve properly set the pull request status:
  • In case it's not yet "ready for review", I've set it to "draft".
  • In case it's "ready for review", I've changed it from "draft" to "non-draft".

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
75.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[seaona]

This works perfect @NicolasMassart 🔥 It will be very helpful. Thank you so much!!
I've approved it since the change is small and code make sense to me, feel free to merge or ask for another dev review if it's still needed.

metrics-flushat-1.mp4

[seaona]

why is this marked as Sev-1 though? 🤔