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`.

internal/state/indexer/metrics.gen.go

@@ -0,0 +1,51 @@
+// Code generated by metricsgen. DO NOT EDIT.
+
+package indexer
+
+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{
+		BlockEventsSeconds: prometheus.NewHistogramFrom(stdprometheus.HistogramOpts{
+			Namespace: namespace,
+			Subsystem: MetricsSubsystem,
+			Name:      "block_events_seconds",
+			Help:      "Latency for indexing block events.",
+		}, labels).With(labelsAndValues...),
+		TxEventsSeconds: prometheus.NewHistogramFrom(stdprometheus.HistogramOpts{
+			Namespace: namespace,
+			Subsystem: MetricsSubsystem,
+			Name:      "tx_events_seconds",
+			Help:      "Latency for indexing transaction events.",
+		}, labels).With(labelsAndValues...),
+		BlocksIndexed: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: MetricsSubsystem,
+			Name:      "blocks_indexed",
+			Help:      "Number of complete blocks indexed.",
+		}, labels).With(labelsAndValues...),
+		TransactionsIndexed: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: MetricsSubsystem,
+			Name:      "transactions_indexed",
+			Help:      "Number of transactions indexed.",
+		}, labels).With(labelsAndValues...),
+	}
+}
+
+func NopMetrics() *Metrics {
+	return &Metrics{
+		BlockEventsSeconds:  discard.NewHistogram(),
+		TxEventsSeconds:     discard.NewHistogram(),
+		BlocksIndexed:       discard.NewCounter(),
+		TransactionsIndexed: discard.NewCounter(),
+	}
+}

internal/state/indexer/metrics.go

@@ -2,12 +2,10 @@ package indexer
 
 import (
 	"github.com/go-kit/kit/metrics"
-	"github.com/go-kit/kit/metrics/discard"
-
-	prometheus "github.com/go-kit/kit/metrics/prometheus"
-	stdprometheus "github.com/prometheus/client_golang/prometheus"
 )
 
+//go:generate go run github.com/tendermint/tendermint/scripts/metricsgen -struct=Metrics
+
 // MetricsSubsystem is a the subsystem label for the indexer package.
 const MetricsSubsystem = "indexer"
 
@@ -25,49 +23,3 @@ type Metrics struct {
 	// Number of transactions indexed.
 	TransactionsIndexed metrics.Counter
 }
-
-// PrometheusMetrics returns Metrics build using Prometheus client library.
-// Optionally, labels can be provided along with their values ("foo",
-// "fooValue").
-func PrometheusMetrics(namespace string, labelsAndValues ...string) *Metrics {
-	labels := []string{}
-	for i := 0; i < len(labelsAndValues); i += 2 {
-		labels = append(labels, labelsAndValues[i])
-	}
-	return &Metrics{
-		BlockEventsSeconds: prometheus.NewHistogramFrom(stdprometheus.HistogramOpts{
-			Namespace: namespace,
-			Subsystem: MetricsSubsystem,
-			Name:      "block_events_seconds",
-			Help:      "Latency for indexing block events.",
-		}, labels).With(labelsAndValues...),
-		TxEventsSeconds: prometheus.NewHistogramFrom(stdprometheus.HistogramOpts{
-			Namespace: namespace,
-			Subsystem: MetricsSubsystem,
-			Name:      "tx_events_seconds",
-			Help:      "Latency for indexing transaction events.",
-		}, labels).With(labelsAndValues...),
-		BlocksIndexed: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
-			Namespace: namespace,
-			Subsystem: MetricsSubsystem,
-			Name:      "blocks_indexed",
-			Help:      "Number of complete blocks indexed.",
-		}, labels).With(labelsAndValues...),
-		TransactionsIndexed: prometheus.NewCounterFrom(stdprometheus.CounterOpts{
-			Namespace: namespace,
-			Subsystem: MetricsSubsystem,
-			Name:      "transactions_indexed",
-			Help:      "Number of transactions indexed.",
-		}, labels).With(labelsAndValues...),
-	}
-}
-
-// NopMetrics returns an indexer metrics stub that discards all samples.
-func NopMetrics() *Metrics {
-	return &Metrics{
-		BlockEventsSeconds:  discard.NewHistogram(),
-		TxEventsSeconds:     discard.NewHistogram(),
-		BlocksIndexed:       discard.NewCounter(),
-		TransactionsIndexed: discard.NewCounter(),
-	}
-}