NOTE: This only works for counters, if you're trying to extend this, you should probably switch to using prom-client.
function makePrometheus(stats, defaultLabels = {}) {
    const metricMapping = {
        processed: 'teraslice_slices_processed',
        failed: 'teraslice_slices_failed',
        queued: 'teraslice_slices_queued',
        job_duration: '', // this isn't really useful, omitting
        workers_joined: 'teraslice_workers_joined',
        workers_disconnected: 'teraslice_workers_disconnected',
        workers_reconnected: 'teraslice_workers_reconnected'
    };

    let returnString = '';
    _.forEach(stats.controllers, (value, key) => {
        const name = metricMapping[key];
        if (name !== '') {
            returnString += `# TYPE ${name} counter\n`;
            const labels = makePrometheusLabels(defaultLabels);
            returnString += `${name}${labels} ${value}\n`;
        }
    });
    return returnString;
}