The metrics introduced in #8430 are not clear when no inbound federation is being received from a server:

An inspection of a graph zoomed in shows this being hard to use in action

This however doesn't show up as clearly when looking at a wider scale, and it's not obvious there is an issue - it just looks like it's successfully having a very small amount of lag and the problem has gone away.

Not emitting a metric while there aren't enough messages to provide any data seems like a better option here - at the moment we are emitting the out of date metric only, so any averages / minimums etc will not work correctly.