Add Histogram exemplar support (#310)

Author: alexcardell

core/src/main/scala/io/chrisdavenport/epimetheus/Histogram.scala

@@ -5,6 +5,7 @@ import cats.effect._
 import cats.implicits._
 import io.prometheus.metrics.core.datapoints.DistributionDataPoint
 import io.prometheus.metrics.core.metrics.{Histogram => JHistogram}
+import io.prometheus.metrics.model.snapshots.Labels
 
 import scala.annotation.tailrec
 import scala.concurrent.duration._
@@ -24,6 +25,11 @@ sealed abstract class Histogram[F[_]] {
     */
   def observe(d: Double): F[Unit]
 
+  def observeWithExemplar(d: Double, exemplarLabels: Map[String, String]): F[Unit] 
+
+  def observeWithExemplar(d: Double, exemplarLabels: (String, String)*): F[Unit] = 
+    observeWithExemplar(d, exemplarLabels.toMap)
+
   def mapK[G[_]](fk: F ~> G): Histogram[G] =
     new Histogram.MapKHistogram[F, G](this, fk)
 
@@ -257,6 +263,12 @@ object Histogram {
   ) extends Histogram[F] {
     def observe(d: Double): F[Unit] = Sync[F].delay(underlying.observe(d))
 
+    def observeWithExemplar(d: Double, exemplarLabels: Map[String, String]): F[Unit] = {
+      val labels = exemplarLabels.toList.flatMap { case (k, v) => List(k, v) }
+      val jLabels = Labels.of(labels:_*)
+      Sync[F].delay(underlying.observeWithExemplar(d, jLabels))
+    }
+
     override private[epimetheus] def asJava: F[JHistogram] = underlying.pure[F]
   }
 
@@ -265,6 +277,12 @@ object Histogram {
   ) extends Histogram[F] {
     def observe(d: Double): F[Unit] = Sync[F].delay(underlying.observe(d))
 
+    def observeWithExemplar(d: Double, exemplarLabels: Map[String, String]): F[Unit] = {
+      val labels = exemplarLabels.toList.flatMap { case (k, v) => List(k, v) }
+      val jLabels = Labels.of(labels:_*)
+      Sync[F].delay(underlying.observeWithExemplar(d, jLabels))
+    }
+
     override private[epimetheus] def asJava: F[JHistogram] =
       ApplicativeThrow[F].raiseError(
         new IllegalArgumentException(
@@ -277,8 +295,12 @@ object Histogram {
       private[Histogram] val base: Histogram[F],
       fk: F ~> G
   ) extends Histogram[G] {
+
     def observe(d: Double): G[Unit] = fk(base.observe(d))
 
+    override def observeWithExemplar(d: Double, exemplarLabels: Map[String,String]): G[Unit] = 
+      fk(base.observeWithExemplar(d, exemplarLabels))
+
     override private[epimetheus] def asJava: G[JHistogram] = fk(base.asJava)
   }
 

core/src/test/scala/io/chrisdavenport/epimetheus/HistogramSpec.scala

@@ -20,4 +20,35 @@ class HistogramSpec extends munit.CatsEffectSuite {
 
     test.attempt.map(_.isRight).assert
   }
+
+  test("Histogram: observe with exemplars (map)") {
+    val setup = for {
+      cr <- PrometheusRegistry.build[IO]
+      h <- Histogram.noLabels(cr, Name("boo"), "Boo ")
+    } yield h
+
+    val test = for {
+      h <- setup
+      _ <- h.observeWithExemplar(0.1, Map("exemplar1" -> "value1"))
+    } yield ()
+
+    test.attempt.map(_.isRight).assert
+
+  }
+
+  test("Histogram: observe with exemplars (tuples)") {
+    val setup = for {
+      cr <- PrometheusRegistry.build[IO]
+      h <- Histogram.noLabels(cr, Name("boo"), "Boo ")
+    } yield h
+
+    val test = for {
+      h <- setup
+      _ <- h.observeWithExemplar(0.1, "exemplar1" -> "value1")
+    } yield ()
+
+    test.attempt.map(_.isRight).assert
+
+  }
+
 }