[SPARK-32026][CORE][TEST] Add PrometheusServletSuite

### What changes were proposed in this pull request?

This PR aims to add `PrometheusServletSuite`.

### Why are the changes needed?

This improves the test coverage.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Pass the newly added test suite.

Closes #28865 from erenavsarogullari/spark_driver_prometheus_metrics_improvement.

Authored-by: Eren Avsarogullari <erenavsarogullari@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

core/src/test/scala/org/apache/spark/metrics/sink/PrometheusServletSuite.scala

@@ -0,0 +1,73 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.metrics.sink
+
+import java.util.Properties
+
+import scala.collection.JavaConverters._
+
+import com.codahale.metrics.{Counter, Gauge, MetricRegistry}
+import org.scalatest.PrivateMethodTester
+
+import org.apache.spark.SparkFunSuite
+
+class PrometheusServletSuite extends SparkFunSuite with PrivateMethodTester {
+
+  test("register metrics") {
+    val sink = createPrometheusServlet()
+
+    val gauge = new Gauge[Double] {
+      override def getValue: Double = 5.0
+    }
+
+    val counter = new Counter
+    counter.inc(10)
+
+    sink.registry.register("gauge1", gauge)
+    sink.registry.register("gauge2", gauge)
+    sink.registry.register("counter1", counter)
+
+    val metricGaugeKeys = sink.registry.getGauges.keySet.asScala
+    assert(metricGaugeKeys.equals(Set("gauge1", "gauge2")),
+      "Should contain 2 gauges metrics registered")
+
+    val metricCounterKeys = sink.registry.getCounters.keySet.asScala
+    assert(metricCounterKeys.equals(Set("counter1")),
+      "Should contain 1 counter metric registered")
+
+    val gaugeValues = sink.registry.getGauges.values.asScala
+    assert(gaugeValues.size == 2)
+    gaugeValues.foreach(gauge => assert(gauge.getValue == 5.0))
+
+    val counterValues = sink.registry.getCounters.values.asScala
+    assert(counterValues.size == 1)
+    counterValues.foreach(counter => assert(counter.getCount == 10))
+  }
+
+  test("normalize key") {
+    val key = "local-1592132938718.driver.LiveListenerBus." +
+      "listenerProcessingTime.org.apache.spark.HeartbeatReceiver"
+    val sink = createPrometheusServlet()
+    val suffix = sink invokePrivate PrivateMethod[String]('normalizeKey)(key)
+    assert(suffix == "metrics_local_1592132938718_driver_LiveListenerBus_" +
+      "listenerProcessingTime_org_apache_spark_HeartbeatReceiver_")
+  }
+
+  private def createPrometheusServlet(): PrometheusServlet =
+    new PrometheusServlet(new Properties, new MetricRegistry, securityMgr = null)
+}