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/metrics_test.go

@@ -0,0 +1,63 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package udprelay
+
+import (
+	"slices"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"tailscale.com/util/usermetric"
+)
+
+func TestMetrics(t *testing.T) {
+	c := qt.New(t)
+	deregisterMetrics()
+	r := &usermetric.Registry{}
+	m := registerMetrics(r)
+
+	// Expect certain prom names registered.
+	have := r.MetricNames()
+	want := []string{
+		"tailscaled_peer_relay_forwarded_packets_total",
+		"tailscaled_peer_relay_forwarded_bytes_total",
+		"tailscaled_peer_relay_endpoints_total",
+	}
+	slices.Sort(have)
+	slices.Sort(want)
+	c.Assert(have, qt.CmpEquals(), want)
+
+	// Validate addEndpoints.
+	m.addEndpoints(1)
+	c.Assert(m.endpoints.Value(), qt.Equals, int64(1))
+	c.Assert(cMetricEndpoints.Value(), qt.Equals, int64(1))
+	m.addEndpoints(-1)
+	c.Assert(m.endpoints.Value(), qt.Equals, int64(0))
+	c.Assert(cMetricEndpoints.Value(), qt.Equals, int64(0))
+
+	// Validate countForwarded.
+	m.countForwarded(true, true, 1, 1)
+	c.Assert(m.forwarded44Bytes.Value(), qt.Equals, int64(1))
+	c.Assert(m.forwarded44Packets.Value(), qt.Equals, int64(1))
+	c.Assert(cMetricForwarded44Bytes.Value(), qt.Equals, int64(1))
+	c.Assert(cMetricForwarded44Packets.Value(), qt.Equals, int64(1))
+
+	m.countForwarded(true, false, 2, 2)
+	c.Assert(m.forwarded46Bytes.Value(), qt.Equals, int64(2))
+	c.Assert(m.forwarded46Packets.Value(), qt.Equals, int64(2))
+	c.Assert(cMetricForwarded46Bytes.Value(), qt.Equals, int64(2))
+	c.Assert(cMetricForwarded46Packets.Value(), qt.Equals, int64(2))
+
+	m.countForwarded(false, true, 3, 3)
+	c.Assert(m.forwarded64Bytes.Value(), qt.Equals, int64(3))
+	c.Assert(m.forwarded64Packets.Value(), qt.Equals, int64(3))
+	c.Assert(cMetricForwarded64Bytes.Value(), qt.Equals, int64(3))
+	c.Assert(cMetricForwarded64Packets.Value(), qt.Equals, int64(3))
+
+	m.countForwarded(false, false, 4, 4)
+	c.Assert(m.forwarded66Bytes.Value(), qt.Equals, int64(4))
+	c.Assert(m.forwarded66Packets.Value(), qt.Equals, int64(4))
+	c.Assert(cMetricForwarded66Bytes.Value(), qt.Equals, int64(4))
+	c.Assert(cMetricForwarded66Packets.Value(), qt.Equals, int64(4))
+}