On the system bus, particularly when using LSMs, it's useful for monitors to be able to see the credentials of D-Bus connections.

It's also useful for monitors to be able to "catch up" with the details of who owns which names.

dbus-daemon could follow this logic:

on a new connection {
    inject a Monitoring.ConnectionInfo signal into all monitors
}

when a connection becomes a monitor {
    for each existing connection {
        inject a Monitoring.ConnectionInfo signal into that monitor, as if the
        connection had just connected
    }
}

where Monitoring.ConnectionInfo is something like this:

    o.fd.DBus.Monitoring.ConnectionInfo(s, s, as, a{sv})
        s: unique_name
        s: debug_string
            a human-readable, not necessarily trustworthy representation
            of the connection
            (implementation: this is basically
            BusConnectionData->cached_loginfo_string)
        as: well_known_names
        a{sv}: same things returned by GetConnectionCredentials

dbus-monitor could either display these by default, or have a command-line option to display them.

For efficiency, ConnectionInfo should not actually be sent on the real bus, just sent to monitors.