metricsgen: port metrics code generation tool (#9156)

This is a port of #8470 and #8501 from the former mainline branch, in
support of #2600 and #9076. 

No additional changes other than checking out the code have been
implemented in this PR. A subsequent PR will incorporate this code.

scripts/metricsgen/testdata/commented/metrics.gen.go

@@ -0,0 +1,30 @@
+// Code generated by metricsgen. DO NOT EDIT.
+
+package commented
+
+import (
+	"github.com/go-kit/kit/metrics/discard"
+	prometheus "github.com/go-kit/kit/metrics/prometheus"
+	stdprometheus "github.com/prometheus/client_golang/prometheus"
+)
+
+func PrometheusMetrics(namespace string, labelsAndValues ...string) *Metrics {
+	labels := []string{}
+	for i := 0; i < len(labelsAndValues); i += 2 {
+		labels = append(labels, labelsAndValues[i])
+	}
+	return &Metrics{
+		Field: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: MetricsSubsystem,
+			Name:      "field",
+			Help:      "Height of the chain. We expect multi-line comments to parse correctly.",
+		}, labels).With(labelsAndValues...),
+	}
+}
+
+func NopMetrics() *Metrics {
+	return &Metrics{
+		Field: discard.NewGauge(),
+	}
+}

scripts/metricsgen/testdata/commented/metrics.go

@@ -0,0 +1,11 @@
+package commented
+
+import "github.com/go-kit/kit/metrics"
+
+//go:generate go run ../../../../scripts/metricsgen -struct=Metrics
+
+type Metrics struct {
+	// Height of the chain.
+	// We expect multi-line comments to parse correctly.
+	Field metrics.Gauge
+}