Function relay_event 

pub async fn relay_event(
    client: &Client,
    upstream: &str,
    headers: &HeaderMap,
    peer: SocketAddr,
    body: Bytes,
) -> Response

Forwards a Plausible event, carrying the real visitor IP.

X-Forwarded-For is not optional: without it Plausible’s bot filter rejects proxied events outright, because every one of them would appear to originate from this server.