Public leptos_meta::register function

Is your feature request related to a problem? Please describe.

Currently I inject css into head via Style in SSR mode, and control css via custom logic in Hydrate mode.

fn mount_style(id: &str, content: &'static str) {
    if #[cfg(feature = "ssr")] {
        let _ = view! {
            <Style id=id>
                {content}
            </Style>
        };
    } else {
        let head = document().head().expect("head no exist");
        let style = head
            .query_selector(&format!("style#{id}"))
            .expect("query style element error");
        if style.is_some() {
            return;
        }
        let style = document()
            .create_element("style")
            .expect("create style element error");
        _ = style.set_attribute("id", &id);
        style.set_text_content(Some(content));
        _ = head.prepend_with_node_1(&style);
    }
}

When I hydrate via the following code, <Style>". b {}"</Style> gets hydrated to the <style>". a{}"</style> element instead of the <style>". b{}"</style> element.

#[component]
pub fn Button() -> impl IntoView {
    mount_style("button", ".a {}");
    view! {
        <Style>
             ".b {}"
         </Style>
    }
}

Describe the solution you'd like

Public leptos_meta::register function. Allows passing a second parameter to control the position of the generated HTML string to avoid hydration.

pub fn register<E, At, Ch>(
    el: HtmlElement<E, At, Ch>,
    config: RegisterConfig,
) -> RegisteredMetaTag<E, At, Ch>

pub struct RegisterConfig {
    position: ElementPosition
}

pub enum ElementPosition {
    // Before the <!--HEAD--> comment.
    MarkerBefore,
    // After the <!--HEAD--> comment.
    MarkerAfter,
}

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context

RegisterConfig is a struct in order to expand more fields in the future to achieve the same functions as unhead.

It seems like there are multiple people interested in having the ability to inject things into the <head> where leptos_meta does it, while also modifying the behavior of the relevant components pretty signficantly. (See #2856, #3188) It would be helpful to unite these different ideas into one set of extensions to leptos_meta.

I think at least #2856 and this seem to result from the fact that the pre-0.7 leptos_meta components have side effects — simply creating a <Style>, without every using it in the view that is returned, injects something into the <head>. It seems like your implementation above relies on this behavior. (i.e., mount_style does not return a view that should be rendered, but () — it is purely side effects)

I don't think I fully understand either the use case for what you're trying to do, or what you're asking for, though.

leptos-rs / leptos

Public leptos_meta::register function #3230