scripts/metricsgen: add the initial version of metricsgen (#8479)

This pull requests adds a new tool, metricsgen, for generating Tendermint metrics constructors from `Metrics` struct definitions. This tool aims to reduce the amount of boilerplate required to add additional metrics to Tendermint.

Its working is fairly simple, it parses the go ast, extracts field information, and uses this field information to execute a go template. 

This pull request also adds a proof-of-concept of the tool's output and working by using it to generate the [indexer metrics](https://github.com/tendermint/tendermint/pull/8479/files#diff-4b0c597b6fa05332a2f9a8e0ce079e360602942fae99dc5485f1edfe71c0a29e) using `//go:generate` directives and a simple `make` target.

The next steps for this tool are documented in https://github.com/tendermint/tendermint/issues/8485 and https://github.com/tendermint/tendermint/issues/8486, which detail using the tool to generate the `metrics.md` documentation file and using the tool to migrate away from `go-kit`.

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

@@ -0,0 +1,30 @@
+// Code generated by metricsgen. DO NOT EDIT.
+
+package basic
+
+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{
+		Height: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: MetricsSubsystem,
+			Name:      "height",
+			Help:      "simple metric that tracks the height of the chain.",
+		}, labels).With(labelsAndValues...),
+	}
+}
+
+func NopMetrics() *Metrics {
+	return &Metrics{
+		Height: discard.NewGauge(),
+	}
+}

scripts/metricsgen/testdata/basic/metrics.go

@@ -0,0 +1,11 @@
+package basic
+
+import "github.com/go-kit/kit/metrics"
+
+//go:generate go run ../../../../scripts/metricsgen -struct=Metrics
+
+// Metrics contains metrics exposed by this package.
+type Metrics struct {
+	// simple metric that tracks the height of the chain.
+	Height metrics.Gauge
+}