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>
