Use a custom collector to avoid checking labels

In order to avoid checking label sets stay the same across all the
emitted metrics, use a custom collector instead of going over one of
prometheus' provided ones. Because of the use case (create a registry,
discard it after one time), this probably also reduces the resource
usage.

The reason for this workaround is that in case of failure, the emitted
metrics have different labelsets. Specifically, labels describing the
error are added. If we end up with a mix of successful and unsuccessful
requests, prometheus will reject one of the metrics because they are
using different sets of labels.

Reevaluate this later.

Signed-off-by: Marcelo E. Magallon <marcelo.magallon@grafana.com>

internal/k6runner/k6runner.go

@@ -85,7 +85,26 @@ func (r Script) Run(ctx context.Context, registry *prometheus.Registry, logger l
 	return nil
 }
 
+type customCollector struct {
+	metrics []prometheus.Metric
+}
+
+func (c *customCollector) Describe(ch chan<- *prometheus.Desc) {
+	// We do not send any descriptions in order to create an unchecked
+	// metric. This allows us to have a metric with the same name and
+	// different label values.
+	//
+	// TODO(mem): reevaluate if this is really want we want to do.
+}
+
+func (c *customCollector) Collect(ch chan<- prometheus.Metric) {
+	for _, m := range c.metrics {
+		ch <- m
+	}
+}
+
 func textToRegistry(metrics []byte, registry prometheus.Registerer, logger zerolog.Logger) error {
+	collector := &customCollector{}
 	promDecoder := expfmt.NewDecoder(bytes.NewBuffer(metrics), expfmt.FmtText)
 	decoderOpts := expfmt.DecodeOptions{Timestamp: model.Now()}
 	for {
@@ -117,21 +136,23 @@ func textToRegistry(metrics []byte, registry prometheus.Registerer, logger zerol
 
 				delete(sample.Metric, model.MetricNameLabel)
 
-				g := prometheus.NewGauge(prometheus.GaugeOpts{
-					Name:        string(name),
-					ConstLabels: metricToLabels(sample.Metric),
-				})
-
-				err := registry.Register(g)
+				desc := prometheus.NewDesc(string(name), mf.GetHelp(), nil, metricToLabels(sample.Metric))
+				// TODO(mem): maybe change this to untyped?
+				m, err := prometheus.NewConstMetric(desc, prometheus.GaugeValue, float64(sample.Value))
 				if err != nil {
+					logger.Error().Err(err).Msg("creating prometheus metric")
 					return err
 				}
 
-				g.Set(float64(sample.Value))
+				collector.metrics = append(collector.metrics, m)
 			}
 
 		case io.EOF:
 			// nothing was returned, we are done
+			if err := registry.Register(collector); err != nil {
+				return err
+			}
+
 			return nil
 
 		default: