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/metricsdiff/metricsdiff_test.go

@@ -0,0 +1,62 @@
+package main_test
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	metricsdiff "github.com/tendermint/tendermint/scripts/metricsgen/metricsdiff"
+)
+
+func TestDiff(t *testing.T) {
+	for _, tc := range []struct {
+		name      string
+		aContents string
+		bContents string
+
+		want string
+	}{
+		{
+			name: "labels",
+			aContents: `
+			metric_one{label_one="content", label_two="content"} 0
+			`,
+			bContents: `
+			metric_one{label_three="content", label_four="content"} 0
+			`,
+			want: `Label changes:
+Metric: metric_one
++++ label_three
++++ label_four
+--- label_one
+--- label_two
+`,
+		},
+		{
+			name: "metrics",
+			aContents: `
+			metric_one{label_one="content"} 0
+			`,
+			bContents: `
+			metric_two{label_two="content"} 0
+			`,
+			want: `Metric changes:
++++ metric_two
+--- metric_one
+`,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			bufA := bytes.NewBuffer([]byte{})
+			bufB := bytes.NewBuffer([]byte{})
+			_, err := io.WriteString(bufA, tc.aContents)
+			require.NoError(t, err)
+			_, err = io.WriteString(bufB, tc.bContents)
+			require.NoError(t, err)
+			md, err := metricsdiff.DiffFromReaders(bufA, bufB)
+			require.NoError(t, err)
+			require.Equal(t, tc.want, md.String())
+		})
+	}
+}