Visualization interval scaled warning doesn't display correct interval

[jen-huang]

Kibana version: 6.4 branch, master

Steps to reproduce:

1. Visualize > Create > Vertical Bars
2. Set time filter to last 15 days
3. Add Date Histogram buckets, set interval to Millisecond, run
4. Note that the interval warning shows that it has been scaled to 12 hours, but in reality it was scaled to 3 hours:

5. Refresh the page, note that the interval warning now has a different erroneous message:

Expected behavior: Interval warning should display the actual interval it was scaled to.

Any additional context: This is seen on the 6.4 branch as well as master.

cc @elastic/kibana-app

[sulemanof]

This was fixed by #34991 in 7.2.0