tsweb: split promvarz into an optional dependency

Allows the use of tsweb without pulling in all of the heavy prometheus
client libraries, protobuf and so on.

Updates #15160

Signed-off-by: David Anderson <dave@tailscale.com>

tsweb/promvarz/promvarz_test.go

@@ -23,7 +23,7 @@ func TestHandler(t *testing.T) {
 	testVar1.Set(42)
 	testVar2.Set(4242)
 
-	svr := httptest.NewServer(http.HandlerFunc(Handler))
+	svr := httptest.NewServer(http.HandlerFunc(handler))
 	defer svr.Close()
 
 	want := `