Add layer tag value: mq.

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/context/tag/Tags.java

@@ -51,6 +51,7 @@ public final class Tags {
         private static final String DB_LAYER = "db";
         private static final String RPC_FRAMEWORK_LAYER = "rpc";
         private static final String HTTP_LAYER = "http";
+        private static final String MQ_LAYER = "mq";
 
         public static void asDB(AbstractSpan span) {
             SPAN_LAYER_TAG.set(span, DB_LAYER);
@@ -63,6 +64,10 @@ public final class Tags {
         public static void asHttp(AbstractSpan span) {
             SPAN_LAYER_TAG.set(span, HTTP_LAYER);
         }
+
+        public static void asMQ(AbstractSpan span) {
+            SPAN_LAYER_TAG.set(span, MQ_LAYER);
+        }
     }
 
     /**