[#29] metrics: Support dump descriptions

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

metrics/desc_test.go

@@ -0,0 +1,26 @@
+//go:build dump_metrics
+
+package metrics
+
+import (
+	"encoding/json"
+	"flag"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+var metricsPath = flag.String("out", "", "File to export http gateway metrics to.")
+
+func TestDescribeAll(t *testing.T) {
+	flag.Parse()
+
+	require.NotEmpty(t, metricsPath, "flag 'out' must be provided to dump metrics description")
+
+	data, err := json.Marshal(DescribeAll())
+	require.NoError(t, err)
+
+	err = os.WriteFile(*metricsPath, data, 0644)
+	require.NoError(t, err)
+}