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())
+}

tsnet/tsnet.go

@@ -126,6 +126,7 @@ type Server struct {
 	initOnce         sync.Once
 	initErr          error
 	lb               *ipnlocal.LocalBackend
+	sys              *tsd.System
 	netstack         *netstack.Impl
 	netMon           *netmon.Monitor
 	rootPath         string // the state directory
@@ -518,6 +519,7 @@ func (s *Server) start() (reterr error) {
 	}
 
 	sys := new(tsd.System)
+	s.sys = sys
 	if err := s.startLogger(&closePool, sys.HealthTracker(), tsLogf); err != nil {
 		return err
 	}
@@ -1227,6 +1229,13 @@ func (s *Server) CapturePcap(ctx context.Context, pcapFile string) error {
 	return nil
 }
 
+// Sys returns a handle to the Tailscale subsystems of this node.
+//
+// This is not a stable API, nor are the APIs of the returned subsystems.
+func (s *Server) Sys() *tsd.System {
+	return s.sys
+}
+
 type listenKey struct {
 	network string
 	host    netip.Addr // or zero value for unspecified