VMware PowerCLI allows viewing performance statistics collected by vCenter on vSphere Datacenters, Clusters, ESXi Hosts, VMs, and datastores. The cmdlet to be used is "Get-Stat."
Get-Stat -Entity <cluster> -Stat Running against a vSphere cluster would reveal all available cluster metrics and their current values.
You can also choose to pull statistics corresponding to a specific metric. For example, you could just fetch the memory usage average numbers. To do so, use the corresponding MetricId using the -Stat switch.
You can use PowerShell to further transform the statistics into a desired form - for instance, the memory consumption average for the last seven days. To do so, pass the output of the Get-Stat command to the Measure-Object cmdlet.
Running against a vSphere Host would also reveal a similar set of metrics and values corresponding to ESXi. And much like with the cluster stats, you could filter and extract the metrics/value you are after.
To list all available metrics for a specific vCenter managed object, run the Get-StatType -Entity "Name of the object"
