Is length extension attack considered as collision?
No, the length extension attack/property is not considered a collision. It does not allow to build a collision.

The length extension property is that given the hash of a bitstring $M$ of given length $l$ (but arbitrary and unknown content), it is possible to compute the hash of $M\mathbin\|F(l)\mathbin\|E$ with $F(l)$ a short bitstring deduced from the length of $l$, and any known bitstring $E$.

It follows that if messages $M$ and $M'$ of the same length collide (have the same hash even though $M\ne M'$), then $M\mathbin\|F(l)\mathbin\|E$ and $M'\mathbin\|F(l)\mathbin\|E$ also collide. Therefore, the length extension property eases building more collisions from an existing collision among messages of the same length.

MD5, SHA-1, SHA-256, SHA-512 have the length extension property (and correspondingly, it is used to extend MD5 and SHA-1 collisions). SHA-3 and SHA-512/256 do not have the length extension property, because enough of the internal state does not make it to the output. The situation is not clear cut for other SHA-2 hashes truncated to more than half their internal state size (e.g. SHA-224 has only 32 bits of resistance to length extension).