PMM Upgrade via UI results in many errors and users being logged out during the upgrade

(It was fixed in RC and dev-latest)

Based on the result logs of running upgrades across several PMM versions, in this example, you will see a gif for 2.18.0 to 2.27.0 but has been reproduced on 2.15.1 -> 2.27.0 also. 

User Impact: User is logged out of the UI while an upgrade is getting executed, they can't log in, when they refresh the page, they see the home page but with a lot of errors and no data.

User can see everything running after several mins of waiting, around 10 mins of wait time when the Server starts running and showing monitoring data again. 

STR:
1) Setup PMM-Server 2.18.0 or 2.19.0 or older version

2) Enable Experimental Repo

3) Make sure you have a client node connected with atleast 1-2 DB services being monitored.

4) Let the instance run for 2-3 hours before triggering upgrade, run upgrade on UI, and observe the error messages and users being logged out of the UI.