Agent Stop working after Upgrade PMM-Server to 2.18.0

On further investigation, we found that not only custom queries but any metrics from the agent stop hitting PMM-Server after the upgrade, this issue below is just one example of how we discovered it on one of the upgrade tests

*Original Issue was discovered when Custom Metrics tests failed*

The issue observed after the upgrade to 2.18.0, metrics for custom queries have stopped being collected. 

Impact on User:

If the user has set up for custom queries and a custom dashboard built on top of these, it will stop working 

STR: 

1) Setup PMM 2.17.0 and a client instance (pmm-client 2.17.0) with custom queries, use this post as a reference if you are doing this for the first time

2) Examples can be used from https://github.com/Percona-Lab/pmm-custom-queries/tree/master/mysql in our testing setup we are using "ps-memory-summary.yml"

3) Let it run for about 20 mins, check if you are able to get data for metrics:  

mysql_performance_schema_memory_summary_current_bytes

4) enable testing/experimental Repo and upgrade PMM-Server to 2.18.0 and let the client still run 2.17.0. 
5) Check the metrics, you will see no data for custom metrics post-upgrade. 

Observation :

On Upgrading the client to 2.18.0, the custom metrics started hitting again.