Custom Queries on pmm-agent inside pmm-server won't be available post docker-way upgrade

Impact on User 

Customer using some custom queries for remote instances, ex. Postgresql, RDS etc., if they perform a docker-way upgrade the custom query files are deleted.

STR:

Just enable custom queries for mysql, pgsql..for pmm-agent & exporter running inside pmm-server docker container. 

Perform Docker-way Upgrade. 

Expected Results:

Custom Query files are not lost and survive docker-way upgrade as they do the UI upgrade. 

Actual Results:

Custom Query files are delete when user perform docker-way upgrade. 

We probably should add a note about such use cases in our documentation, informing users about how it can impact their deployments.