[Prometheus Emitter] Add to code coverage and remove code smell

extensions-contrib/prometheus-emitter/src/main/java/org/apache/druid/emitter/prometheus/Metrics.java

@@ -64,7 +64,7 @@ public DimensionsAndCollector getByName(String name, String service)
 
   public Metrics(String namespace, String path, boolean isAddHostAsLabel, boolean isAddServiceAsLabel, Map<String, String> extraLabels)
   {
-    Map<String, DimensionsAndCollector> registeredMetrics = new HashMap<>();
+    Map<String, DimensionsAndCollector> parsedRegisteredMetrics = new HashMap<>();
     Map<String, Metric> metrics = readConfig(path);
 
     if (extraLabels == null) {
@@ -116,10 +116,10 @@ public Metrics(String namespace, String path, boolean isAddHostAsLabel, boolean
       }
 
       if (collector != null) {
-        registeredMetrics.put(name, new DimensionsAndCollector(dimensions, collector, metric.conversionFactor));
+        parsedRegisteredMetrics.put(name, new DimensionsAndCollector(dimensions, collector, metric.conversionFactor));
       }
     }
-    this.registeredMetrics = Collections.unmodifiableMap(registeredMetrics);
+    this.registeredMetrics = Collections.unmodifiableMap(parsedRegisteredMetrics);
   }
 
   private Map<String, Metric> readConfig(String path)

extensions-contrib/prometheus-emitter/src/test/java/org/apache/druid/emitter/prometheus/MetricsTest.java

@@ -20,6 +20,7 @@
 package org.apache.druid.emitter.prometheus;
 
 import io.prometheus.client.Histogram;
+import org.apache.druid.java.util.common.ISE;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -93,4 +94,20 @@ public void testMetricsConfigurationWithBadExtraLabels()
 
     Assert.assertTrue(actualMessage.contains(expectedMessage));
   }
+
+  @Test
+  public void testMetricsConfigurationWithNonExistentMetric()
+  {
+    Metrics metrics = new Metrics("test_4", null, true, true, null);
+    DimensionsAndCollector nonExistentDimsCollector = metrics.getByName("non/existent", "historical");
+    Assert.assertNull(nonExistentDimsCollector);
+  }
+
+  @Test
+  public void testMetricsConfigurationWithUnSupportedType()
+  {
+    Assert.assertThrows(ISE.class, () -> {
+      new Metrics("test_5", "src/test/resources/defaultMetricsTest.json", true, true, null);
+    });
+  }
 }

extensions-contrib/prometheus-emitter/src/test/resources/defaultMetricsTest.json

@@ -0,0 +1,3 @@
+{
+  "query/nonExistent" : { "dimensions" : ["dataSource"], "type" : "nonExistent", "help":  "Non supported type."}
+}