I don't know how to interpret the row values and the greyed "backend" row.
For example I don't have any clue about what the difference between these two rows is.
Link(haproxy entry): https://imgur.com/a/9559mf9
Link(haproxy green row): https://imgur.com/sv2fPQ6
Link(haproxy grey row backend): https://imgur.com/H2U8xn6
I can't send any pictures due to being new and therefor I added the links above.
As shown in the pictures the values differ.
The rows lit up in green are systems that are actively responding to HAProxy, and that are available to load balance services between.
Backend is the cumulative total between all services available. The differences are when you add weighted differences between one system and another, and global configuration limits.
In your instance, it looks like your backend is rate limited under that of what your Prometheus configuration is built for.