wgengine/magicsock: track per-endpoint changes in ringbuffer

This change adds a ringbuffer to each magicsock endpoint that keeps a
fixed set of "changes"–debug information about what updates have been
made to that endpoint.

Additionally, this adds a LocalAPI endpoint and associated
"debug peer-status" CLI subcommand to fetch the set of changes for a given
IP or hostname.

Updates tailscale/corp#9364

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I34f726a71bddd0dfa36ec05ebafffb24f6e0516a

ipn/ipnlocal/local.go

@@ -4897,3 +4897,25 @@ func (b *LocalBackend) StreamDebugCapture(ctx context.Context, w io.Writer) erro
 	}
 	return nil
 }
+
+func (b *LocalBackend) GetPeerEndpointChanges(ctx context.Context, ip netip.Addr) ([]magicsock.EndpointChange, error) {
+	pip, ok := b.e.PeerForIP(ip)
+	if !ok {
+		return nil, fmt.Errorf("no matching peer")
+	}
+	if pip.IsSelf {
+		return nil, fmt.Errorf("%v is local Tailscale IP", ip)
+	}
+	peer := pip.Node
+
+	mc, err := b.magicConn()
+	if err != nil {
+		return nil, fmt.Errorf("getting magicsock conn: %w", err)
+	}
+
+	chs, err := mc.GetEndpointChanges(peer)
+	if err != nil {
+		return nil, fmt.Errorf("getting endpoint changes: %w", err)
+	}
+	return chs, nil
+}