Akka Http Prometheus Endpoint

App.scala

import io.prometheus.client.CollectorRegistry
import io.prometheus.client.dropwizard.DropwizardExports
import io.prometheus.client.hotspot.DefaultExports
import nl.grons.metrics.scala.DefaultInstrumented
import akka.actor.ActorSystem
import akka.stream.ActorMaterializer
import akka.http.scaladsl.Http

object App extends App with DefaultInstrumented {
 
  // Add JVM metrics exporters to default registry
  DefaultExports.initialize()
  
  // Add Dropwizard adapter exporter
  CollectorRegistry.defaultRegistry.register(new DropwizardExports(metricRegistry))
  
  val prometheusEndpoint = PrometheusEndpoint(CollectorRegistry.defaultRegistry)
  
  implicit val system = ActorSystem("system")
  implicit val materializer = ActorMaterializer()
  
  Http().bindAndHandle(prometheusEndpoint, "0.0.0.0", 9000)
}

build.sbt

lazy val akkaHttpVersion = "10.0.3"
lazy val metricsScalaVersion = "3.5.6"
lazy val prometheusVersion = "0.0.26"

libraryDependencies ++= Seq(
  // Akka Http
  "com.typesafe.akka" %% "akka-http" % akkaHttpVersion,
  
  // Scala API for dropwizard metrics
  "nl.grons" %% "metrics-scala" % metricsScalaVersion,
  
  // Provides exporters for HotSpot JVM metrics from MBeans
  "io.prometheus" % "simpleclient_hotspot" % prometheusVersion, 
  // Provides exporter that works as an adapter between Dropwizard and Prometheus registries
  "io.prometheus" % "simpleclient_dropwizard" % prometheusVersion,  
  
  // Common stuff that would help us with encoding response in Prometheus format.
  "io.prometheus" % "simpleclient_common" % prometheusVersion
)

PrometheusEndpoint.scala

import java.io.StringWriter
import akka.http.scaladsl.model._
import akka.http.scaladsl.server.Directives._
import akka.http.scaladsl.server.Route
import io.prometheus.client.CollectorRegistry
import io.prometheus.client.exporter.common.TextFormat
import scala.collection.JavaConverters._

object PrometheusEndpoint {
  // Custom Prometheus content type. See https://prometheus.io/docs/instrumenting/exposition_formats
  private val `text/plain; version=0.0.4; charset=utf-8` = ContentType {
    MediaType.customWithFixedCharset(
      "text",
      "plain",
      HttpCharsets.`UTF-8`,
      params = Map("version" -> "0.0.4")
    )
  }
  
  private def renderMetrics(registry: CollectorRegistry, names: Set[String]): String = {
    val writer = new StringWriter()
    TextFormat.write004(writer, registry.filteredMetricFamilySamples(names.toSet.asJava))
    writer.toString
  }
  
  def apply(registry: CollectorRegistry): Route =
    // We gonna serve at `/metrics/prometheus` and use `name` parameter as filter if provided by scraper
    (get & path("metrics" / "prometheus") & parameter('name.*)) { names =>
      val content = renderMetrics(registry, names.toSet)
      complete {
        HttpResponse(entity = HttpEntity(`text/plain; version=0.0.4; charset=utf-8`, content))
      }
    }
}