When I play Astral Heron to a battlefield, does my next card this turn get reduced by 2 energy and 2 power?

Yes — if Astral Heron is the first card you play that turn, your next card costs [2] Energy and [A][A] (2 Power, any domain) less.

Heron counts as your first card; once it's finalized at a battlefield, its trigger sees that condition satisfied and fires — the discount then applies to the next (second) card you play. It never discounts itself.

Caveat: if you already played another card earlier that turn, Heron won't trigger — the "first card" event already passed, and Heron wasn't at a battlefield when it was evaluated (FAQ #11863, #11952).