Intern MBean names used in cache

Use string interning when creating the "matchName" used
in the caching data structure, in order to reuse
previously created String objects and to avoid
duplication of Strings with identical contents in memory.

collector/src/main/java/io/prometheus/jmx/JmxCollector.java

@@ -581,7 +581,7 @@ public void recordBean(
                     attributeName = attrName;
                 }
 
-                String matchName = beanName + attributeName + ": " + matchBeanValue;
+                String matchName = (beanName + attributeName + ": " + matchBeanValue).intern();
 
                 if (rule.cache) {
                     MatchedRule cachedRule = config.rulesCache.get(rule, matchName);