Improve MemberToMetricMappings a bit, add javadoc

implementation/src/main/java/io/smallrye/metrics/MemberToMetricMappings.java

@@ -27,6 +27,15 @@
 
 import io.smallrye.metrics.elementdesc.MemberInfo;
 
+/**
+ * This class represents mappings between Java methods and the set of metric IDs
+ * associated with them. This is computed once at boot/build time to avoid having to
+ * do runtime reflection on every invocation of the relevant methods.
+ *
+ * This class does NOT use thread-safe map implementations, so populating the mappings
+ * must only be performed by one thread. Querying the mappings later at runtime can be done
+ * concurrently.
+ */
 public class MemberToMetricMappings {
 
     MemberToMetricMappings() {
@@ -37,11 +46,11 @@ public class MemberToMetricMappings {
         simpleTimers = new HashMap<>();
     }
 
-    private Map<MemberInfo, Set<MetricID>> counters;
-    private Map<MemberInfo, Set<MetricID>> concurrentGauges;
-    private Map<MemberInfo, Set<MetricID>> meters;
-    private Map<MemberInfo, Set<MetricID>> timers;
-    private Map<MemberInfo, Set<MetricID>> simpleTimers;
+    private final Map<MemberInfo, Set<MetricID>> counters;
+    private final Map<MemberInfo, Set<MetricID>> concurrentGauges;
+    private final Map<MemberInfo, Set<MetricID>> meters;
+    private final Map<MemberInfo, Set<MetricID>> timers;
+    private final Map<MemberInfo, Set<MetricID>> simpleTimers;
 
     public Set<MetricID> getCounters(MemberInfo member) {
         return counters.get(member);