Maybe too obvious given line 2, but I've managed to make it fit in some interesting and cryptic ways!
I am fast like an arrow but slow like a turtle
You can play violin music both quickly (allegro) and slowly (adagio).
I play like a Cello but with a silence echo
A violin certainly plays like a cello; I'm not sure what "with a silence echo" means though.
I am long like a Metro but not quite the rainbow
Buskers might be found on the Metro playing a violin. A violin bow is not quite the same word as "rainbow".
Give me an answer that fit this riddle!
"fit this riddle" is a Spoonerism of "rit this fiddle", and although "rit" is meaningless, a fiddle is another word for a violin!