lib/promrelabel/debug: make metric name easier to distinguish from labels

Previously, users could get confused in cases when metric name which is valid for MetricsQL but not valid for Prom was used in relabeling debug. See #8584 for example.

This adds highlight to a metric name and prints empty labels set in order to explicitly show metric name and set of labels.

Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>

lib/promrelabel/debug.qtpl

@@ -185,7 +185,7 @@ function submitRelabelDebugForm(e) {
                 {% else %}
                         {%s metricName %}
                 {% endif %}
-                {% if len(labelsList) == 0 %}{% return %}{% endif %}
+                {% if len(labelsList) == 0 %}{}{% return %}{% endif %}
         {% endif %}
 {
         {% for i, label := range labelsList %}
@@ -201,7 +201,7 @@ function submitRelabelDebugForm(e) {
 
 {% func mustFormatLabels(s string) %}
         {% code labels := promutil.MustNewLabelsFromString(s) %}
-        {%= labelsWithHighlight(labels, nil, "") %}
+        {%= labelsWithHighlight(labels, map[string]struct{}{"__name__": {}}, "") %}
 {% endfunc %}
 
 {% endstripspace %}

lib/promrelabel/debug.qtpl.go

@@ -464,6 +464,8 @@ func streamlabelsWithHighlight(qw422016 *qt422016.Writer, labels *promutil.Label
 		}
 //line lib/promrelabel/debug.qtpl:188
 		if len(labelsList) == 0 {
+//line lib/promrelabel/debug.qtpl:188
+			qw422016.N().S(`{}`)
 //line lib/promrelabel/debug.qtpl:188
 			return
 //line lib/promrelabel/debug.qtpl:188
@@ -547,7 +549,7 @@ func streammustFormatLabels(qw422016 *qt422016.Writer, s string) {
 	labels := promutil.MustNewLabelsFromString(s)
 
 //line lib/promrelabel/debug.qtpl:204
-	streamlabelsWithHighlight(qw422016, labels, nil, "")
+	streamlabelsWithHighlight(qw422016, labels, map[string]struct{}{"__name__": {}}, "")
 //line lib/promrelabel/debug.qtpl:205
 }