You can use PowerShell Get-AzMetric for that. See $SpaceMetric on below script.
$StorageAccounts = Get-AzResource -ResourceType Microsoft.Sql/managedInstances
FOREACH($StorageAccount in $StorageAccounts){
$ResourceId = $StorageAccount.ResourceId
$Start = (get-date).AddDays(-7)
$End = get-date
$SpaceMetric = Get-AzMetric -ResourceId $ResourceId -MetricName "storage_space_used_mb" -WarningAction Ignore
$IOMetric = Get-AzMetric -ResourceId $ResourceId -MetricName "io_requests" -StartTime $Start -EndTime $End -WarningAction Ignore
$CoreMetric = Get-AzMetric -ResourceId $ResourceId -MetricName "virtual_core_count" -WarningAction Ignore
#Load Variables for SQL Statement.
$TeamGroupName = $SubscriptionID.TeamGroupsName
$ServiceName = $SubscriptionID.ServiceName
$SubscriptionName = $SubscriptionID.SubscriptionName
$SubID = $SubscriptionID.SubscriptionID
$StorageAccountName = $StorageAccount.Name
$StorageType = 'SQL-MI'
$SQLMISize = ($SpaceMetric.Data | Select-Object -First 1).Average/1024
$TotalRequests = ($IOMetric.Data | Select-Object -First 1).Average
$CoreCount = ($CoreMetric.Data | Select-Object -First 1).Average
$Region = $StorageAccount.Location
$RefreshedAt = Get-Date