Fix the parsing of threshold name tags containing tokens

@efdknittlfrank pointed out that v0.38.0 presented a bug when thresholds
were defined for URL grouping. Indeed, using colons and curly braces
inside of a thresholds tags definition would confuse the parser, and
lead to an unclear error.

This commit ensures that we parse thresholds tags properly:
* closing curly brace is now parsed from the right
* we ensure the closing curly brace is the last character in the string.
* knowing the position of the opening an closing tag definition curly
braces, we extract the whole definition's substring based on that.

metrics/metric.go

@@ -133,7 +133,7 @@ var ErrMetricNameParsing = errors.New("parsing metric name failed")
 // of "key:value" strings. On failure, it returns an error containing the `ErrMetricNameParsing` in its chain.
 func ParseMetricName(name string) (string, []string, error) {
 	openingTokenPos := strings.IndexByte(name, '{')
-	closingTokenPos := strings.IndexByte(name, '}')
+	closingTokenPos := strings.LastIndexByte(name, '}')
 	containsOpeningToken := openingTokenPos != -1
 	containsClosingToken := closingTokenPos != -1
 
@@ -150,25 +150,29 @@ func ParseMetricName(name string) (string, []string, error) {
 		return "", nil, fmt.Errorf("%w; reason: unmatched opening/close curly brace", ErrMetricNameParsing)
 	}
 
+	// If the closing brace token appears before the opening one,
+	// the expression is malformed
 	if closingTokenPos < openingTokenPos {
 		return "", nil, fmt.Errorf("%w; reason: closing curly brace appears before opening one", ErrMetricNameParsing)
 	}
 
-	parserFn := func(c rune) bool {
-		return c == '{' || c == '}'
+	// If the last character is not a closing brace token,
+	// the expression is malformed.
+	if closingTokenPos != (len(name) - 1) {
+		err := fmt.Errorf(
+			"%w; reason: missing closing curly brace in last position"+
+				"of the threshold expression",
+			ErrMetricNameParsing,
+		)
+		return "", nil, err
 	}
 
-	// Split the metric_name{tag_key:tag_value,...} expression
-	// into two "metric_name" and "tag_key:tag_value,..." strings.
-	parts := strings.FieldsFunc(name, parserFn)
-	if len(parts) == 0 || len(parts) > 2 {
-		return "", nil, ErrMetricNameParsing
-	}
-
-	// Split the tag key values
-	tags := strings.Split(parts[1], ",")
+	// We already know the position of the opening and closing curly brace
+	// tokens. Thus, we extract the string in between them, and split its
+	// content to obtain the tags key values.
+	tags := strings.Split(name[openingTokenPos+1:closingTokenPos], ",")
 
-	// For each tag definition, ensure
+	// For each tag definition, ensure it is correctly formed
 	for i, t := range tags {
 		keyValue := strings.SplitN(t, ":", 2)
 
@@ -179,5 +183,5 @@ func ParseMetricName(name string) (string, []string, error) {
 		tags[i] = strings.TrimSpace(t)
 	}
 
-	return parts[0], tags, nil
+	return name[0:openingTokenPos], tags, nil
 }

metrics/metric_test.go

@@ -111,6 +111,20 @@ func TestParseMetricName(t *testing.T) {
 			wantTags:             []string{"group:::mygroup"},
 			wantErr:              false,
 		},
+		{
+			name:                 "metric name with valid name and repeated curly braces tokens in tags definition",
+			metricNameExpression: "http_req_duration{name:http://${}.com}",
+			wantMetricName:       "http_req_duration",
+			wantTags:             []string{"name:http://${}.com"},
+			wantErr:              false,
+		},
+		{
+			name:                 "metric name with valid name and repeated curly braces and colon tokens in tags definition",
+			metricNameExpression: "http_req_duration{name:http://${}.com,url:ssh://github.com:grafana/k6}",
+			wantMetricName:       "http_req_duration",
+			wantTags:             []string{"name:http://${}.com", "url:ssh://github.com:grafana/k6"},
+			wantErr:              false,
+		},
 		{
 			name:                 "metric name with tag definition missing `:value`",
 			metricNameExpression: "test_metric{easyas}",
@@ -146,6 +160,11 @@ func TestParseMetricName(t *testing.T) {
 			metricNameExpression: "test_metric}abc{bar",
 			wantErr:              true,
 		},
+		{
+			name:                 "metric name with valid name and trailing characters after closing curly brace in tags definition",
+			metricNameExpression: "test_metric{foo:ba}r",
+			wantErr:              true,
+		},
 	}
 	for _, tt := range tests {
 		tt := tt