SLING-6702 - Make MetricsService accessible as easily as a Logger

Add a note around handling rewiring of metric service later on
diff --git a/src/main/java/org/apache/sling/commons/metrics/internal/MetricsServiceImpl.java b/src/main/java/org/apache/sling/commons/metrics/internal/MetricsServiceImpl.java
index 5d57ab9..6d5fd08 100644
--- a/src/main/java/org/apache/sling/commons/metrics/internal/MetricsServiceImpl.java
+++ b/src/main/java/org/apache/sling/commons/metrics/internal/MetricsServiceImpl.java
@@ -61,6 +61,11 @@
 
     private JmxReporter reporter;
 
+    /*  WARNING: if any OSGi configuration parameters are added to this class
+     *  we’ll need to handle the service restart case to prevent service
+     *  references retrieved using MetricsServiceFactory from becoming
+     *  stale. See discussion in SLING-6702.
+     */
     @Activate
     private void activate(BundleContext context, Map<String, Object> config) {
         enableJMXReporter();