Expose metrics in prometheus format

build.sbt

@@ -102,6 +102,9 @@ lazy val registry = (project in file("schemer-registry"))
       quill,
       quillAsyncPostgres,
       flyway,
+      prometheusClient,
+      prometheusClientCommon,
+      prometheusHotspot,
       scalaTest
     ),
     excludeDependencies ++= Seq(

project/Dependencies.scala

@@ -20,6 +20,10 @@ object Dependencies {
     ExclusionRule("javax.mail")
   }
 
+  lazy val prometheusClient = "io.prometheus" % "simpleclient" % "0.2.0"
+  lazy val prometheusClientCommon = "io.prometheus" % "simpleclient_common" % "0.2.0"
+  lazy val prometheusHotspot = "io.prometheus" % "simpleclient_hotspot" % "0.2.0"
+
   lazy val akkaHttpCore = "com.typesafe.akka" %% "akka-http-core" % Versions.akkaHttpVersion
   lazy val akkaHttp = "com.typesafe.akka" %% "akka-http" % Versions.akkaHttpVersion
   lazy val sprayJsonAkka = "com.typesafe.akka" %% "akka-http-spray-json" % Versions.akkaHttpVersion

schemer-registry/src/main/scala/schemer/registry/routes/HealthRoutes.scala

@@ -1,13 +1,41 @@
 package schemer.registry.routes
 
-import akka.http.scaladsl.server.Directives.{complete, get, path}
+import java.io.{StringWriter, Writer}
+import java.util
+
+import akka.http.scaladsl.model.{HttpCharsets, HttpEntity, MediaType}
+import akka.http.scaladsl.server.Directives._
+import io.prometheus.client.Collector.MetricFamilySamples
+import io.prometheus.client.CollectorRegistry
+import io.prometheus.client.exporter.common.TextFormat
+import io.prometheus.client.hotspot.{DefaultExports, StandardExports}
 
 trait HealthRoutes {
+
+  DefaultExports.initialize()
+  val collectorRegistry       = CollectorRegistry.defaultRegistry
+  private val mediaTypeParams = Map("version" -> "0.0.4")
+  private val mediaType =
+    MediaType.customWithFixedCharset("text", "plain", HttpCharsets.`UTF-8`, params = mediaTypeParams)
+
+  def toPrometheusTextFormat(e: util.Enumeration[MetricFamilySamples]): String = {
+    val writer: Writer = new StringWriter()
+    TextFormat.write004(writer, e)
+
+    writer.toString
+  }
+
   val healthRoutes = path("health") {
     get {
       complete {
         "OK"
       }
     }
+  } ~ path("metrics") {
+    get {
+      complete {
+        HttpEntity(mediaType, toPrometheusTextFormat(collectorRegistry.metricFamilySamples()))
+      }
+    }
   }
 }