Add Stringer to Query interface

Signed-off-by: darshanime <deathbullet@gmail.com>

promql/engine.go

@@ -117,6 +117,8 @@ type Query interface {
 	Stats() *stats.QueryTimers
 	// Cancel signals that a running query execution should be aborted.
 	Cancel()
+	// String returns the original query string.
+	String() string
 }
 
 // query implements the Query interface.
@@ -141,10 +143,17 @@ type query struct {
 type QueryOrigin struct{}
 
 // Statement implements the Query interface.
+// Calling this after Exec may result in panic,
+// see https://github.com/prometheus/prometheus/issues/8949.
 func (q *query) Statement() parser.Statement {
 	return q.stmt
 }
 
+// String implements the Query interface.
+func (q *query) String() string {
+	return q.q
+}
+
 // Stats implements the Query interface.
 func (q *query) Stats() *stats.QueryTimers {
 	return q.stats