Add CommandTag to Rows interface

This allows handling queries where it is unknown if there will be a
result set or not. If it is not a result set returning query the
command tag will still be available.

pgxpool/rows.go

@@ -1,6 +1,7 @@
 package pgxpool
 
 import (
+	"github.com/jackc/pgconn"
 	"github.com/jackc/pgproto3/v2"
 	"github.com/jackc/pgx/v4"
 )
@@ -11,6 +12,7 @@ type errRows struct {
 
 func (errRows) Close()                                         {}
 func (e errRows) Err() error                                   { return e.err }
+func (errRows) CommandTag() pgconn.CommandTag                  { return nil }
 func (errRows) FieldDescriptions() []pgproto3.FieldDescription { return nil }
 func (errRows) Next() bool                                     { return false }
 func (e errRows) Scan(dest ...interface{}) error               { return e.err }
@@ -43,6 +45,10 @@ func (rows *poolRows) Err() error {
 	return rows.r.Err()
 }
 
+func (rows *poolRows) CommandTag() pgconn.CommandTag {
+	return rows.CommandTag()
+}
+
 func (rows *poolRows) FieldDescriptions() []pgproto3.FieldDescription {
 	return rows.r.FieldDescriptions()
 }