derp: add server version to /debug, expvars

This will make it easier for a human to tell what
version is deployed, for (say) correlating line numbers
in profiles or panics to corresponding source code.

It'll also let us observe version changes in prometheus.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/derper/derper.go

@@ -33,6 +33,7 @@ import (
 	"tailscale.com/net/stun"
 	"tailscale.com/tsweb"
 	"tailscale.com/types/key"
+	"tailscale.com/version"
 )
 
 var (
@@ -221,6 +222,7 @@ func debugHandler(s *derp.Server) http.Handler {
 		f("<li><b>Hostname:</b> %v</li>\n", *hostname)
 		f("<li><b>Uptime:</b> %v</li>\n", tsweb.Uptime())
 		f("<li><b>Mesh Key:</b> %v</li>\n", s.HasMeshKey())
+		f("<li><b>Version:</b> %v</li>\n", version.LONG)
 
 		f(`<li><a href="/debug/vars">/debug/vars</a> (Go)</li>
    <li><a href="/debug/varz">/debug/varz</a> (Prometheus)</li>

derp/derp_server.go

@@ -29,6 +29,7 @@ import (
 	"tailscale.com/metrics"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
+	"tailscale.com/version"
 )
 
 var debug, _ = strconv.ParseBool(os.Getenv("DERP_DEBUG_LOGS"))
@@ -1191,6 +1192,9 @@ func (s *Server) ExpVar() expvar.Var {
 	m.Set("multiforwarder_created", &s.multiForwarderCreated)
 	m.Set("multiforwarder_deleted", &s.multiForwarderDeleted)
 	m.Set("packet_forwarder_delete_other_value", &s.removePktForwardOther)
+	var expvarVersion expvar.String
+	expvarVersion.Set(version.LONG)
+	m.Set("version", &expvarVersion)
 	return m
 }