pkg/textparse: handle trailing labels comma (#2752)

pkg/textparse/lex.l

@@ -73,7 +73,7 @@ M     [a-zA-Z_:]
                                       l.offsets = append(l.offsets, l.i)
 
 <lstateLabels>[ \t]+
-<lstateLabels>\}                      s = lstateValue
+<lstateLabels>,?\}                    s = lstateValue
                                       l.mend = l.i
 <lstateLabels>,?                      s = lstateLName
                                       l.offsets = append(l.offsets, l.i)

pkg/textparse/lex.l.go

@@ -264,7 +264,12 @@ yystate20:
 
 yystate21:
 	c = l.next()
-	goto yyrule8
+	switch {
+	default:
+		goto yyrule8
+	case c == '}':
+		goto yystate22
+	}
 
 yystate22:
 	c = l.next()
@@ -391,7 +396,7 @@ yyrule5: // {S}({M}|{D})*
 yyrule6: // [ \t]+
 
 	goto yystate0
-yyrule7: // \}
+yyrule7: // ,?\}
 	{
 		s = lstateValue
 		l.mend = l.i

pkg/textparse/parse_test.go

@@ -31,7 +31,7 @@ func TestParse(t *testing.T) {
 	input := `# HELP go_gc_duration_seconds A summary of the GC invocation durations.
 # TYPE go_gc_duration_seconds summary
 go_gc_duration_seconds{quantile="0"} 4.9351e-05
-go_gc_duration_seconds{quantile="0.25"} 7.424100000000001e-05
+go_gc_duration_seconds{quantile="0.25",} 7.424100000000001e-05
 go_gc_duration_seconds{quantile="0.5",a="b"} 8.3835e-05
 go_gc_duration_seconds_count 99
 some:aggregate:rate5m{a_b="c"}	1
@@ -52,7 +52,7 @@ go_goroutines 33  	123123`
 			v:    4.9351e-05,
 			lset: labels.FromStrings("__name__", "go_gc_duration_seconds", "quantile", "0"),
 		}, {
-			m:    `go_gc_duration_seconds{quantile="0.25"}`,
+			m:    `go_gc_duration_seconds{quantile="0.25",}`,
 			v:    7.424100000000001e-05,
 			lset: labels.FromStrings("__name__", "go_gc_duration_seconds", "quantile", "0.25"),
 		}, {