cmd/printmetric: add start of tool to dump usermetrics to JSON

Updates tailscale/corp#22075

Change-Id: I5b539fcb4aca1b93406cf139c719a5e3c64ff7f7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/printmetric/printmetric.go

@@ -0,0 +1,39 @@
+// The printmetric command prints out JSON of the usermetric definitions.
+package main
+
+import (
+	"io/ioutil"
+	"log"
+	"net/http/httptest"
+	"os"
+
+	"tailscale.com/ipn/store/mem"
+	"tailscale.com/tsnet"
+	"tailscale.com/tstest/integration/testcontrol"
+)
+
+func main() {
+	var control testcontrol.Server
+	ts := httptest.NewServer(&control)
+	defer ts.Close()
+
+	td, err := ioutil.TempDir("", "testcontrol")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(td)
+
+	tsn := &tsnet.Server{
+		Dir:        td,
+		Store:      new(mem.Store),
+		UserLogf:   log.Printf,
+		Ephemeral:  true,
+		ControlURL: ts.URL,
+	}
+	if err := tsn.Start(); err != nil {
+		log.Fatal(err)
+	}
+	rec := httptest.NewRecorder()
+	tsn.Sys().UserMetricsRegistry().Handler(rec, httptest.NewRequest("GET", "/unused", nil))
+	os.Stdout.Write(rec.Body.Bytes())
+}