Add comments for MQE tracing (#134)

diff --git a/common.graphqls b/common.graphqls
index 98bd655..5d5ec16 100644
--- a/common.graphqls
+++ b/common.graphqls
@@ -134,7 +134,7 @@
     startTime: Long!
     endTime: Long!
     duration: Long!
-    msg: String!
+    msg: String
     error: String
diff --git a/metrics-v3.graphqls b/metrics-v3.graphqls
index 173c0b0..c4fd56a 100644
--- a/metrics-v3.graphqls
+++ b/metrics-v3.graphqls
@@ -99,5 +99,7 @@
     # Get the list of all available metrics in the current OAP server.
     # Param, regex, could be used to filter the metrics by name.
     listMetrics(regex: String): [MetricDefinition!]!
+    # Param, if debug is true will enable the query tracing and return DebuggingTrace in the ExpressionResult.
+    # Param, if dumpDBRsp is true the database response will dump into the DebuggingTrace span message.
     execExpression(expression: String!, entity: Entity!, duration: Duration!, debug: Boolean, dumpDBRsp: Boolean): ExpressionResult!