as it is now possible to reset the leaderboard during development.
You can only reset player progress data for your draft leaderboards.
To reset leaderboards in the Google Play Developer Console, click the button labeled Reset leaderboard progress at the bottom of the form for that event. To reset leaderboard data programmatically, call the Management API Scores methods. Reference: `https://developers.google.com/games/services/common/concepts/leaderboards#resetting_a_leaderboard`

Updating the answer as it is now possible to reset the leaderboard during development.
> You can only reset player progress data for your draft leaderboards. > > To reset leaderboards in the Google Play Developer Console, click the > button labeled Reset leaderboard progress at the bottom of the form > for that event. To reset leaderboard data programmatically, call the > Management API Scores methods.

Reference: https://developers.google.com/games/services/common/concepts/leaderboards#resetting_a_leaderboard