Fixed few warnings

Author: robsunday

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/yaml/JmxRule.java

@@ -47,6 +47,7 @@ public class JmxRule extends MetricStructure {
   @Nullable private String prefix;
   private Map<String, Metric> mapping;
 
+  @Nullable
   public String getBean() {
     return bean;
   }
@@ -90,6 +91,7 @@ private String validatePrefix(String prefix) {
     return prefix;
   }
 
+  @Nullable
   public String getPrefix() {
     return prefix;
   }

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/yaml/Metric.java

@@ -21,6 +21,7 @@ public class Metric extends MetricStructure {
   @Nullable private String metric;
   @Nullable private String desc;
 
+  @Nullable
   public String getMetric() {
     return metric;
   }
@@ -35,6 +36,7 @@ private String validateMetricName(String name) {
     return name;
   }
 
+  @Nullable
   public String getDesc() {
     return desc;
   }