add trace logging

performanceDb/src/main/java/org/lucares/performance/db/Group.java

@@ -26,4 +26,9 @@ class Group {
 	public void addFile(final PdbFile file) {
 		files.add(file);
 	}
+	
+	@Override
+	public String toString() {
+		return tags + ": " + files.size()+" files";
+	}
 }

performanceDb/src/main/java/org/lucares/performance/db/Grouping.java

@@ -8,9 +8,12 @@ import java.util.List;
 import java.util.Map;
 
 import org.lucares.pdb.api.Tags;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class Grouping {
-
+	private static final Logger LOGGER = LoggerFactory.getLogger(Grouping.class);
+	
 	public static final List<String> NO_GROUPING = Collections.emptyList();
 
 	private final List<Group> groups = new ArrayList<>();
@@ -42,6 +45,7 @@ public class Grouping {
 			}
 			result = new Grouping(grouping.values());
 		}
+		LOGGER.trace("grouped {} files by {}: {}", pdbFiles.size(), groupByField, result);
 		return result;
 	}
 
@@ -60,4 +64,9 @@ public class Grouping {
 	public Collection<Group> getGroups() {
 		return groups;
 	}
+	
+	@Override
+	public String toString() {
+		return String.valueOf(groups);
+	}
 }