if I play hostile takeover as my first card to gain control of Astral Heron at a battlefield do I get to use it's effect to make my second card cheaper?

No. Astral Heron's trigger condition is evaluated the moment your first card is played — and at that moment you don't control Heron yet, so the ability can't fire for you.

Hostile Takeover only resolves after you've already played your first card, so gaining control of Heron afterward doesn't retroactively create the discount. The "when you play your first card" trigger looks at who controls Heron when that card is played (Rule 383.2.a.1, 383.2.c), not after. Same principle as the ruling that playing your first card while Heron is at Base, then moving him to a battlefield, doesn't give you the discount — the condition check has already passed.

Note the errata'd text: "the next card you play this turn" — even if you did control Heron, the discount would only apply to a card played after the trigger resolves, and it would expire at end of turn.