これでL.Control.Opacityは使える
//MIERUNE MONO
const m_mono = new L.tileLayer("https://tile.mierune.co.jp/mierune_mono/{z}/{x}/{y}.png", {
attribution: "Maptiles by <a href='http://mierune.co.jp/' target='_blank'>MIERUNE</a>, under CC BY. Data by <a href='http://osm.org/copyright' target='_blank'>OpenStreetMap</a> contributors, under ODbL."
});
m_mono.addTo(map);
L.control.opacity(m_mono, {label: "mierune mono"});