Add class for additional custom attributes

gradle.properties

@@ -1,3 +1,3 @@
 group=com.github.ankurcha
 name=gcloud-logging-slf4j-logback
-version=1.1.11
+version=1.2.0

src/main/java/com/google/cloud/logging/CustomAttributes.java

@@ -0,0 +1,17 @@
+package com.google.cloud.logging;
+
+import java.util.Map;
+
+public class CustomAttributes {
+
+    private final Map<String, Object> attributes;
+
+    public Map<String, Object> getAttributes() {
+        return attributes;
+    }
+
+    public CustomAttributes(Map<String, Object> attributes) {
+        this.attributes = attributes;
+    }
+
+}

src/main/java/com/google/cloud/logging/GoogleCloudLoggingV2Layout.java

@@ -121,6 +121,13 @@ protected Map toJsonMap(ILoggingEvent event) {
                     if (arg instanceof HttpRequestContext) {
                         reqCtx = (HttpRequestContext) arg;
                     }
+
+                    if (arg instanceof CustomAttributes) {
+                        Map<String, Object> attributes = ((CustomAttributes) arg).getAttributes();
+                        if (attributes != null) {
+                            builder.putAll(attributes);
+                        }
+                    }
                 }
             }