net/udprelay: expose peer relay metrics (#18218)

Adding both user and client metrics for peer relay forwarded bytes and
packets, and the total endpoints gauge.

User metrics:
tailscaled_peer_relay_forwarded_packets_total{transport_in, transport_out}
tailscaled_peer_relay_forwarded_bytes_total{transport_in, transport_out}
tailscaled_peer_relay_endpoints_total{}

Where the transport labels can be of "udp4" or "udp6".

Client metrics:
udprelay_forwarded_(packets|bytes)_udp(4|6)_udp(4|6)
udprelay_endpoints

RELNOTE: Expose tailscaled metrics for peer relay.

Updates tailscale/corp#30820

Change-Id: I1a905d15bdc5ee84e28017e0b93210e2d9660259

Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>

net/udprelay/server_test.go

@@ -21,6 +21,7 @@ import (
 	"tailscale.com/tstime/mono"
 	"tailscale.com/types/key"
 	"tailscale.com/types/views"
+	"tailscale.com/util/usermetric"
 )
 
 type testClient struct {
@@ -209,7 +210,9 @@ func TestServer(t *testing.T) {
 
 	for _, tt := range cases {
 		t.Run(tt.name, func(t *testing.T) {
-			server, err := NewServer(t.Logf, 0, true)
+			reg := new(usermetric.Registry)
+			deregisterMetrics()
+			server, err := NewServer(t.Logf, 0, true, reg)
 			if err != nil {
 				t.Fatal(err)
 			}