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.go

@@ -0,0 +1,50 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package prober
+
+import (
+	"slices"
+	"sync"
+)
+
+// histogram serves as an adapter to the Prometheus histogram datatype.
+// The prober framework passes labels at custom metric collection time that
+// it expects to be coupled with the returned metrics. See ProbeClass.Metrics
+// and its call sites. Native prometheus histograms cannot be collected while
+// injecting more labels. Instead we use this type and pass observations +
+// collection labels to prometheus.MustNewConstHistogram() at prometheus
+// metric collection time.
+type histogram struct {
+	count          uint64
+	sum            float64
+	buckets        []float64
+	bucketedCounts map[float64]uint64
+	mx             sync.Mutex
+}
+
+// newHistogram constructs a histogram that buckets data based on the given
+// slice of upper bounds.
+func newHistogram(buckets []float64) *histogram {
+	slices.Sort(buckets)
+	return &histogram{
+		buckets:        buckets,
+		bucketedCounts: make(map[float64]uint64, len(buckets)),
+	}
+}
+
+func (h *histogram) add(v float64) {
+	h.mx.Lock()
+	defer h.mx.Unlock()
+
+	h.count++
+	h.sum += v
+
+	for _, b := range h.buckets {
+		if v > b {
+			continue
+		}
+		h.bucketedCounts[b] += 1
+		break
+	}
+}