prober: plumb a now-required netmon to derphttp

Updates #11896

Change-Id: Ie2f9cd024d85b51087d297aa36c14a9b8a2b8129
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

prober/derp.go

@@ -23,6 +23,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/stun"
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
@@ -544,7 +545,7 @@ func newConn(ctx context.Context, dm *tailcfg.DERPMap, n *tailcfg.DERPNode, isPr
 		return !strings.Contains(s, "derphttp.Client.Connect: connecting to")
 	})
 	priv := key.NewNode()
-	dc := derphttp.NewRegionClient(priv, l, nil /* no netMon */, func() *tailcfg.DERPRegion {
+	dc := derphttp.NewRegionClient(priv, l, netmon.NewStatic(), func() *tailcfg.DERPRegion {
 		rid := n.RegionID
 		return &tailcfg.DERPRegion{
 			RegionID:   rid,