constructor ({ port, logger, registry }) {
    this._port = port
    this._logger = logger
    this._registry = registry

    this._app = express()
    this._app.get('/metrics', (req, res) => {
      res.set('Content-Type', Prometheus.register.contentType)
      res.end(this._registry.metrics())
    })
  }
Create Metrics Server
@param {number} port  - the port to listen on
@param {Object} logger - Logger for logging stuff
@param {Object} register - Prometheus registry that holds metric data