Use real histogram factory in MetricSerialiserTest

Author: amarziali

dd-trace-core/src/test/groovy/datadog/trace/common/metrics/SerializingMetricWriterTest.groovy

@@ -1,5 +1,7 @@
 package datadog.trace.common.metrics
 
+import datadog.metrics.api.Histograms
+import datadog.metrics.impl.DDSketchHistograms
 import datadog.trace.api.Config
 import datadog.trace.api.ProcessTags
 import datadog.trace.api.WellKnownTags
@@ -17,6 +19,18 @@ import static java.util.concurrent.TimeUnit.MILLISECONDS
 import static java.util.concurrent.TimeUnit.SECONDS
 
 class SerializingMetricWriterTest extends DDSpecification {
+
+  static Histograms.Factory originalHistogramFactory
+
+  def setupSpec() {
+    originalHistogramFactory = Histograms.factory
+    Histograms.register(DDSketchHistograms.FACTORY)
+  }
+
+  def cleanupSpec() {
+    Histograms.register(originalHistogramFactory)
+  }
+
   def "should produce correct message #iterationIndex with process tags enabled #withProcessTags" () {
     setup:
     if (withProcessTags) {

products/metrics/metrics-api/src/main/java/datadog/metrics/api/Histograms.java

@@ -16,6 +16,10 @@ public static void register(Factory histograms) {
     }
   }
 
+  static Histograms.Factory getFactory() {
+    return Histograms.factory;
+  }
+
   public interface Factory {
     Histogram newHistogram();