New enhanceLogEntry protected method for overriding (googleapis#1385)

* Issue googleapis#1329

Allow extended classes to access the LogEntry building, so that
additional information may be encoded in each log entry (eg traceid).

Use addLabel rather than setLabels as it avoids the creation and copy
of a HashMap per log entry.

Signed-off-by: Greg Wilkins <gregw@webtide.com>

google-cloud-logging/src/main/java/com/google/cloud/logging/LoggingHandler.java

@@ -26,7 +26,6 @@
 
 import java.util.LinkedList;
 import java.util.List;
-import java.util.Map;
 import java.util.Set;
 import java.util.logging.ErrorManager;
 import java.util.logging.Filter;
@@ -303,15 +302,18 @@ private LogEntry entryFor(LogRecord record) {
       return null;
     }
     Level level = record.getLevel();
-    Map<String, String> labels = ImmutableMap.of(
-        "levelName", level.getName(),
-        "levelValue", String.valueOf(level.intValue()));
-    return LogEntry.newBuilder(Payload.StringPayload.of(payload))
-        .setLabels(labels)
-        .setSeverity(severityFor(level))
-        .build();
+    LogEntry.Builder builder = LogEntry.newBuilder(Payload.StringPayload.of(payload))
+        .addLabel("levelName", level.getName())
+        .addLabel("levelValue", String.valueOf(level.intValue()))
+        .setSeverity(severityFor(level));
+    enhanceLogEntry(builder, record);
+    return builder.build();
   }
-
+
+  protected void enhanceLogEntry(LogEntry.Builder builder, LogRecord record) {
+    // no-op in this class
+  }
+
   private static Severity severityFor(Level level) {
     if (level instanceof LoggingLevel) {
       return ((LoggingLevel) level).getSeverity();