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>

cmd/derpprobe/derpprobe.go

@@ -15,6 +15,9 @@ import (
 	"tailscale.com/prober"
 	"tailscale.com/tsweb"
 	"tailscale.com/version"
+
+	// Support for prometheus varz in tsweb
+	_ "tailscale.com/tsweb/promvarz"
 )
 
 var (