To retrieve the metrics you'll need the Get-Stat cmdlet.
Have a look at my PowerCLI & vSphere statistics – Part 1 – The basics to see how the Get-Stat cmdlet works.
The following is a simple example how this could be done
$start = (Get-Date).AddDays(-7)
$metrics = "cpu.usagemhz.average","cpu.usagemhz.maximum","cpu.usagemhz.minimum"
Get-VM | Get-Stat -Start $start -Stat $metrics |
Select TimeStamp,@{N="VM";E={$_.entity.Name}},MetricId,Value