The Fubini-Study metric $ \omega_{ \mathrm{FS} } $ on the projective space $ \mathbb{P}^1 $

Question

I have a small question about the Fubini-Study metric $ \omega_{ \mathrm{FS} } $ on the projective space $ \mathbb{P}^1 $ appearing in page : $ 119 $ of Daniel Huybrechts's book intiteled : Complex Geometry. An introduction.

In this page of this book, the author says that the fact : $ \int_{ \mathbb{P}^{1} } \omega_{ \mathrm{FS } } = 1 $ implies that $ [ \omega_{ \mathrm{FS} } ] \in H^1 ( \mathbb{P}^1 , \mathbb{Z} ) = \mathbb{Z} $ is a generator. Could you explain to me clearly why do we have this fact ?

Thanks in advance for your help.