The way to say this is:

*It has been raining all day.*

>One continuing event

>We use the present perfect continuous for a single activity that began at a point in the past and is still continuing:

>I’ve been reading your book – it’s great. (I’m still reading it.)

>He’s been living in the village since 1995. (He is still living in the village.)

She has been writing her autobiography since 1987.

https://dictionary.cambridge.org/us/grammar/british-grammar/present-perfect-continuous-i-have-been-working