Add day_zero attribute to Leo most used entry point metric (uplift to 1.94.x)

[brave-builds]

Uplift of #38497
Resolves brave/brave-browser#57603

Pre-approval checklist:

• You have tested your change on Nightly.
• This contains text which needs to be translated.
  • There are more than 7 days before the release.
  • I've notified folks in #l10n on Slack that translations are needed.
• The PR milestones match the branch they are landing to.

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Post-merge checklist:

• The associated issue milestone is set to the smallest version that the changes is landed on.