Auto scaling interval not shown in Discover

[flash1293]

Kibana version: master

Describe the bug: When a manual interval is chosen in the discover histogram which is too small for the current time range, it gets scaled up, but there is no message to tell the user.

Steps to reproduce:

1. Go to discover
2. Expand time range to multiple days
3. Set histogram interval to milliseconds

Expected behavior:

Should state the actual interval (Scaled up to hours)

Screenshots (if relevant):
7.6.1:

master:

I'm only 80% sure this is a bug, did we remove this on purpose?

cc @kertal

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kertal]

@flash1293 will have a look, thx!

[kertal]

This was certainly not removed on purpose, I think it's a leftover of #59958, Visualizations API changed, but the watcher on a changed property wasn't updated. I'm providing a fix