minor code cleanup

Author: SylvainJuge

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/engine/MetricAttribute.java

@@ -5,6 +5,7 @@
 
 package io.opentelemetry.instrumentation.jmx.engine;
 
+import javax.annotation.Nullable;
 import javax.management.MBeanServerConnection;
 import javax.management.ObjectName;
 
@@ -30,7 +31,8 @@ public String getAttributeName() {
     return name;
   }
 
-  String acquireAttributeValue(MBeanServerConnection connection, ObjectName objectName) {
+  @Nullable
+  public String acquireAttributeValue(MBeanServerConnection connection, ObjectName objectName) {
     return extractor.extractValue(connection, objectName);
   }
 }

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/engine/MetricAttributeExtractor.java

@@ -35,7 +35,12 @@ static MetricAttributeExtractor fromObjectNameParameter(String parameterKey) {
     if (parameterKey.isEmpty()) {
       throw new IllegalArgumentException("Empty parameter name");
     }
-    return (dummy, objectName) -> objectName.getKeyProperty(parameterKey);
+    return (dummy, objectName) -> {
+      if (objectName == null) {
+        throw new IllegalArgumentException("missing object name");
+      }
+      return objectName.getKeyProperty(parameterKey);
+    };
   }
 
   static MetricAttributeExtractor fromBeanAttribute(String attributeName) {

instrumentation/jmx-metrics/library/src/test/java/io/opentelemetry/instrumentation/jmx/engine/RuleParserTest.java

@@ -379,7 +379,6 @@ void testConf8() throws Exception {
   @Test
   void testStateMetricConf() throws Exception {
     JmxConfig config = parseConf(CONF9);
-    assertThat(config).isNotNull();
 
     List<JmxRule> rules = config.getRules();
     assertThat(rules).hasSize(1);