DetectFederationLag should tell me what the homeservers in question are

**Is your feature request related to a problem? Please describe.**

When I enable the protection to `DetectFederationLag`, I see messages like these that don't tell me much:

> `Room !REDACTED:REDACTED lag has decreased to an acceptable level. Currently, 0 homeservers are still lagging`

The only thing changing is the number which kept going between 0 to 4.

**Describe the solution you'd like**

I would like to know which are these lagging homeservers, so I would have better idea if this is something actionable upon.

**Describe alternatives you've considered**

Disabling the `DetectFederationLag` protection, which I did.

**Additional context**

I may also not understand the point of `DetectFederationLag`.

Start by locating the `DetectFederationLag` protection and the code that emits the reported lag messages. Read how it tracks lagging homeservers and how its current count is produced; the issue names no files or tests. Done means the messages identify which homeservers are lagging, so the information is actionable.

Written by the indexing model from the issue text.