Postgres Exporter has increased memory usage with pmm-client 2.15.1 & pmm-server 2.16.0

During Release testing another case observed was an increase in memory usage of Postgres exporter on Postgres client instances. 

STR: 

1) Setup PMM-Server 2.15.1
2) Setup PMM-Client 2.15.1, with Postgres instance and let it run for some time. 
3) Docker way Upgrade PMM-Server to 2.16.0 and let the client run 2.15.1 version

You will observe an increase of about 15% in Memory Usage of Postgres exporter on the client instance

4) Let this run for about some hours and then upgrade the client instance to 2.16.0

You will observe another increase of about 15% in Memory Usage of Postgres exporter on the client instance. 

client instance is set up with push mode and running PostgreSQL percona distribution v13 with basic sysbench load and no changes were performed on the client instance in terms of load, everything was the same as with 2.15.1 

it is a relative increase of about 30% as compared to the released 2.15.1 version, based on average values. 

Expected Behaviour: We should investigate the reason for increased memory usage and address it.