cmd/derpprobe,prober: add ability to perform continuous queuing delay measurements against DERP servers

This new type of probe sends DERP packets sized similarly to CallMeMaybe packets
at a rate of 10 packets per second. It records the round-trip times in a Prometheus
histogram. It also keeps track of how many packets are dropped. Packets that fail to
arrive within 5 seconds are considered dropped.

Updates tailscale/corp#24522

Signed-off-by: Percy Wegmann <percy@tailscale.com>

prober/histogram_test.go

@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package prober
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestHistogram(t *testing.T) {
+	h := newHistogram([]float64{1, 2})
+	h.add(0.5)
+	h.add(1)
+	h.add(1.5)
+	h.add(2)
+	h.add(2.5)
+
+	if diff := cmp.Diff(h.count, uint64(5)); diff != "" {
+		t.Errorf("wrong count; (-got+want):%v", diff)
+	}
+	if diff := cmp.Diff(h.sum, 7.5); diff != "" {
+		t.Errorf("wrong sum; (-got+want):%v", diff)
+	}
+	if diff := cmp.Diff(h.bucketedCounts, map[float64]uint64{1: 2, 2: 2}); diff != "" {
+		t.Errorf("wrong bucketedCounts; (-got+want):%v", diff)
+	}
+}