Ask backend to render and write metatile via http. Based on tirex module.
Use lua-resty-http for http requests, so it must be installed in lua_package_path directory.
syntax: result = osm.http.request(map, x, y, z1, z2)
Request enqueue command to fetching map 'map' with x/y/z1 cordination. And also request to render in background between zoom z1 to z2. If request fails return nil.
When z1 == z2, just ask to render single tile.