add logging

hdbarchiver/src/main/java/org/tango/archiving/collector/mode/ArchivingContextList.java

 package org.tango.archiving.collector.mode;
 
 import fr.soleil.tango.archiving.infra.tango.ArchivingConfig;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.util.Collection;
 import java.util.HashSet;
@@ -9,28 +11,30 @@ import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
 public class ArchivingContextList {
+    private final Logger logger = LoggerFactory.getLogger(ArchivingContextList.class);
 
     private final Map<Integer, ArchivingContext> contextList = new ConcurrentHashMap<>();
 
     public final Set<String> pauseContext(int contextId) {
+        Set<String> attributes = new HashSet<>();
         if (contextList.containsKey(contextId)) {
             ArchivingContext context = contextList.get(contextId);
             context.setActive(true);
-            return context.getAttributeConfigMap().keySet();
-        } else {
-            return new HashSet<>();
+            attributes = context.getAttributeConfigMap().keySet();
+            logger.debug("pausing context {} for {}", contextList, attributes);
         }
-
+        return attributes;
     }
 
     public final Collection<ArchivingConfig> resumeContext(int contextId) {
+        Collection<ArchivingConfig> attributes = new HashSet<>();
         if (contextList.containsKey(contextId)) {
             ArchivingContext context = contextList.get(contextId);
             context.setActive(false);
-            return context.getAttributeConfigMap().values();
-        } else {
-            return new HashSet<>();
+            attributes = context.getAttributeConfigMap().values();
+            logger.debug("resuming context {} for {}", contextList, attributes);
         }
+        return attributes;
     }
 
     public final void removeAttribute(String attributeName) {