Play! Framework Controller exposing Prometheus metrics

PrometheusMetricsController.scala

package controllers

import java.io.Writer

import io.prometheus.client.CollectorRegistry
import io.prometheus.client.exporter.common.TextFormat
import play.api.libs.iteratee.Concurrent
import play.api.mvc.{Action, Controller}

/*
 * Don't forget to define a route to this endpoint like:
 *
 * /metrics   controllers.PrometheusMetricsController.index
 *
 */
class PrometheusMetricsController extends Controller {

  def index = Action {
    import play.api.libs.concurrent.Execution.Implicits._
    val responseStream = Concurrent.unicast[Array[Byte]] { channel =>
      val writer = new WriterAdapter(channel)
      TextFormat.write004(writer, CollectorRegistry.defaultRegistry.metricFamilySamples())
      writer.close()
    }

    Ok.stream(responseStream).withHeaders("Content-Type" -> TextFormat.CONTENT_TYPE_004)
  }
}

class WriterAdapter(channel: Concurrent.Channel[Array[Byte]])
  extends Writer {

  override def write(charArray: Array[Char], offset: Int, length: Int): Unit = {
    channel.push(new String(charArray, offset, length).getBytes("UTF-8"))
  }
  override def flush(): Unit = channel.end
  override def close(): Unit = channel.eofAndEnd
}

Thanks Tim. This was helpful. I'm using Play! 2.5 so I had to make a few modifications to make this work with the new Result API.