Dashboards: Size of Temp Files Report Metric value has wrong unit on Postgresql Instance Summary Dashboard

User impact:
The wrong metric unit creates confusion, having such a huge temp file size could reason to worry. 

Original report:
On Setting up Postgresql DB with PMM for monitoring, the Report Size of Temp files on postgresql instance summary dashboard clearly shows the wrong size value for postgresql metric. 

The report seems to show the size in TB's which is not really possible and seems wrong. 

Steps to Reproduce: 

1) Setup postgresql 13 with PMM Server 2.16.0 and run a very basic load on this postgresql DB instance

2) Open Postgresql Instance summary Dashboard after some hours to check the report

3) Size for postgresql is clearly not in the right unit, temp file size can't be in TB since the DB host node can not really make that happen.