Join a relative loki URL to allow prefixes

[kellerkindt]

This allow one to move loki behind a reverse-proxy on another path:

    let (layer, task) = tracing_loki::layer(
        Url::parse("http://127.0.0.1:3100/my/very/other/path/to/loki/").unwrap(),
        vec![("host".into(), "mine".into())].into_iter().collect(),
        vec![].into_iter().collect(),
    )?;

[hrxi]

Thanks. I'll move it to work behind the new builder API only, in order to stay backward-compatible.