ipn/ipnstate: add PeerStatus.KeyExpiry for tailscale status --json

Fixes #6712

Change-Id: I817cd5342fac8a956fcefda2d63158fa488f3395
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnstate/ipnstate.go

@@ -20,6 +20,7 @@ import (
 
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
+	"tailscale.com/types/ptr"
 	"tailscale.com/types/views"
 	"tailscale.com/util/dnsname"
 )
@@ -251,6 +252,10 @@ type PeerStatus struct {
 	// information from control or optimisically set on the client if the
 	// expiration time has passed.
 	Expired bool `json:",omitempty"`
+
+	// KeyExpiry, if present, is the time at which the node key expired or
+	// will expire.
+	KeyExpiry *time.Time `json:",omitempty"`
 }
 
 type StatusBuilder struct {
@@ -435,6 +440,9 @@ func (sb *StatusBuilder) AddPeer(peer key.NodePublic, st *PeerStatus) {
 	if st.Expired {
 		e.Expired = true
 	}
+	if t := st.KeyExpiry; t != nil {
+		e.KeyExpiry = ptr.To(*t)
+	}
 }
 
 type StatusUpdater interface {