WordPress hook for visiting a post

I assume you are looking for a conditional to check if you are on a post page.

You can do so by using is_single():

if (is_single()) {
    // Update your post views here
}

If you insist on using filters or hooks, you can use the_content filter:

add_filter( 'the_content', 'update_post_views' );
function update_post_views($content){
    if (is_single()) {
        $id = get_the_ID();
        // Now update your post's views
    }
    return $content;
}