control/controlclient: add patchify miss stats

Add an opt-in metrics.LabelMap tracking why patchifyPeer fails to
convert a PeersChanged entry into a PeersChangedPatch. The stats are
gated behind the TS_DEBUG_PATCHIFY_PEER_MISS envknob so there is zero
overhead in normal operation.

peerChangeDiff now takes an optional onFalse callback that is called
with the field name on every non-patchable return path. When the
envknob is off, nil is passed and replaced with a no-op at the top of
peerChangeDiff.

The resulting metric renders as:

    counter_patchify_miss{why="Hostinfo"} 2
    counter_patchify_miss{why="peer_not_found"} 1170

Updates tailscale/corp#40088

Change-Id: I2d4b9074bf42ec03ab296c0629a54106bafa873e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/map_debug.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_debug
+
+package controlclient
+
+import "tailscale.com/metrics"
+
+var patchifyMissStats = metrics.NewLabelMap("counter_patchify_miss", "why")
+
+func init() {
+	patchifyMissOnFalse = func(field string) {
+		patchifyMissStats.Add(field, 1)
+	}
+}