Powershell script for a PoSH Grid Widget in a SCOM 2012 R2 Dashboard, lists all RDS Hosts and their last sampled Total Sessions value.

SCOM_RDSH_TotalSession_PoSHWidget.ps1

# Based on https://gallery.technet.microsoft.com/Retrieve-Performance-Data-c4595e2a#content
function Add-Criteria() 
{ 
    param($criteria,$clause) 
     
    if ($criteria -ne $null) 
        {$criteria += " AND $clause"} 
    else 
        {$criteria = $clause} 
         
    return($criteria) 
}

$rdsClass = Get-SCOMClass -Name "Microsoft.Windows.Server.2012.R2.RemoteDesktopServicesRole.Service.RDSessionHost"
$rdsHosts = Get-SCOMMonitoringObject -Class $rdsClass | Sort-Object -Property '[Microsoft.Windows.Computer].PrincipalName'
$rdsTotalSessionRule = Get-SCOMRule -Name "Microsoft.Windows.Server.2012.R2.RemoteDesktopServicesRole.Service.RDSessionHost.PerformanceCollection.TotalSessions"
$objectName = "Terminal Services"
$counterName = "Total Sessions"
$InstanceName = ""


$i=0
foreach ($rdsHost in $rdsHosts) {
    # Build performance criteria
    if ($rdsClass -ne $null) { $criteria = Add-Criteria -criteria $criteria -clause "MonitoringClassId = '{$($rdsClass.Id)}'" } 
    if ($rdsHost -ne $null) { $criteria = Add-Criteria -criteria $criteria -clause "MonitoringObjectFullName = '$($rdsHost.FullName)'" } 
    if ($objectName -ne $null) { $criteria = Add-Criteria -criteria $criteria -clause "ObjectName = '$objectName'" } 
    if ($counterName -ne $null) { $criteria = Add-Criteria -criteria $criteria -clause "CounterName = '$counterName'" } 
    
    #$criteria
    # Get Performance reader object.
    $perfReader = (Get-SCOMManagementGroup).GetMonitoringPerformanceDataReader($criteria)
    while ($perfReader.Read()) {
        $perfData = $perfReader.GetMonitoringPerformanceData()
        $perfValueReader = $perfData.GetValueReader(((Get-Date).AddMinutes(-180)),(Get-Date))

        $perfValues = @()
        while ($perfValueReader.Read()) {
            $perfValues += $perfValueReader.GetMonitoringPerformanceDataValue()
        }
        $perfValue = $perfValues | Sort-Object -Property TimeSampled | select -Last 1
        
        $dataObject = $ScriptContext.CreateInstance("xsd://stegenfeldt/nullschema")
        $dataObject = $ScriptContext.CreateFromObject($rdsHost, "Id=Id,Health=HealthState,Host=Path", $null)
        $dataObject["Time Sampled"] = $perfValue.TimeSampled
        $dataObject["Total Sessions"] = $perfValue.SampleValue
        $ScriptContext.ReturnCollection.Add($dataObject)
    }
    $criteria = $null
    ++$i
}