Revert "Revert promql changes"

This reverts commit 78eb1623ec.

promql/parser/generated_parser.y

@@ -161,7 +161,7 @@ START_METRIC_SELECTOR
 // Type definitions for grammar rules.
 %type <matchers> label_match_list
 %type <matcher> label_matcher
-%type <item> aggregate_op grouping_label match_op maybe_label metric_identifier unary_op at_modifier_preprocessors
+%type <item> aggregate_op grouping_label match_op maybe_label metric_identifier unary_op at_modifier_preprocessors string_identifier
 %type <labels> label_set metric
 %type <lblList> label_set_list
 %type <label> label_set_item
@@ -583,6 +583,10 @@ label_match_list: label_match_list COMMA label_matcher
 
 label_matcher   : IDENTIFIER match_op STRING
                         { $$ = yylex.(*parser).newLabelMatcher($1, $2, $3); }
+                | string_identifier match_op STRING
+                        { $$ = yylex.(*parser).newLabelMatcher($1, $2, $3); }
+                | string_identifier
+                        { $$ = yylex.(*parser).newMetricNameMatcher($1); }
                 | IDENTIFIER match_op error
                         { yylex.(*parser).unexpected("label matching", "string"); $$ = nil}
                 | IDENTIFIER error
@@ -903,6 +907,16 @@ string_literal  : STRING
                         }
                 ;
 
+string_identifier  : STRING
+                        {
+                        $$ = Item{
+                                Typ: METRIC_IDENTIFIER,
+                                Pos: $1.PositionRange().Start,
+                                Val: yylex.(*parser).unquoteString($1.Val),
+                        }
+                        }
+                ;
+
 /*
  * Wrappers for optional arguments.
  */

promql/parser/parse.go

@@ -791,6 +791,18 @@ func (p *parser) checkAST(node Node) (typ ValueType) {
 			// Skip the check for non-empty matchers because an explicit
 			// metric name is a non-empty matcher.
 			break
+		} else {
+			// We also have to make sure a metric name was not set twice inside the
+			// braces.
+			foundMetricName := ""
+			for _, m := range n.LabelMatchers {
+				if m != nil && m.Name == labels.MetricName {
+					if foundMetricName != "" {
+						p.addParseErrf(n.PositionRange(), "metric name must not be set twice: %q or %q", foundMetricName, m.Value)
+					}
+					foundMetricName = m.Value
+				}
+			}
 		}
 
 		// A Vector selector must contain at least one non-empty matcher to prevent
@@ -874,6 +886,15 @@ func (p *parser) newLabelMatcher(label, operator, value Item) *labels.Matcher {
 	return m
 }
 
+func (p *parser) newMetricNameMatcher(value Item) *labels.Matcher {
+	m, err := labels.NewMatcher(labels.MatchEqual, labels.MetricName, value.Val)
+	if err != nil {
+		p.addParseErr(value.PositionRange(), err)
+	}
+
+	return m
+}
+
 // addOffset is used to set the offset in the generated parser.
 func (p *parser) addOffset(e Node, offset time.Duration) {
 	var orgoffsetp *time.Duration