if i have heron at star spring. and play a 2 cost unit to boucne it to base. does that count as play a card while it was at a battlefield?

Yes. Your Heron's ability triggers, even though Star Spring then bounces it to your base.

"When you play your first card each turn, if I'm at a battlefield" checks at the moment the 2-cost unit is played — the Heron is still at Star Spring then, so the condition is satisfied. The bounce happens later, when Star Spring's trigger resolves, and it doesn't cancel the Heron's already-triggered discount. The discount is a pending effect independent of the Heron's location, so your next card still costs [2][A][A] less.

This is exactly the same timing as in FAQ #11683: both triggers fire in the same moment, you control the order they go on the chain, and the bounce doesn't stop the discount. (Rule 383; FAQ 11683.)