Following is the script: (Save with a .vbs extension), I've marked in red the places u need to change so your script will fit your needs.
The 0.8 number means that at 0.8% network utilization, the script will change the adapter's metric and thus lower its priority.


If you just want to change the metric of your network interface from command line,
use this command : (run CMD as admin)


netsh interface ipv4 set interface "wired" metric=5


================================================== ===
On Error Resume Next


strComputer = "."
strMainNIC = "Wired"
strMainNICDevice = "Marvell Yukon 88E8056 PCI-E Gigabit Ethernet Controller"




Set objWMIService = GetObject("winmgmts:" _
& "{impersonationLevel=impersonate}!\\" & strComputer & "\Root\CIMV2" )


Set colItems = objWMIService.ExecQuery _
("Select * From Win32_NetworkAdapter Where NetConnectionID = '" & strMainNIC & "'")


For Each objItem in colItems
strMACAddress = objItem.MACAddress
Wscript.Echo "MACAddress: " & strMACAddress
Next


Set colNetCard = objWMIService.ExecQuery _
("Select * From Win32_NetworkAdapterConfiguration where IPEnabled=TRUE" )


For Each objNetCard in colNetCard
If objNetCard.MACAddress = strMACAddress Then
For Each strIPAddress in objNetCard.IPAddress
Wscript.Echo "Description: " & objNetCard.Description
Wscript.Echo "IP Address: " & strIPAddress
Wscript.Echo "IPConnectionMetric: " & objNetCard.IPConnectionMetric
regValueDataMetric = objNetCard.IPConnectionMetric
Next
End If
Next


For X = 0 to 1000000000000000000


Set objWMIService = GetObject("winmgmts:" _
& "{impersonationLevel=impersonate}!\\" & strComputer & "\Root\CIMV2" )
Set colItems = objWMIService.ExecQuery _
("select * from Win32_PerfFormattedData_TCPIP_NetworkInterface Where Name = '" & strMainNICDevice & "'" )
For Each objItem in colItems
bytesTotal = objitem.BytesTotalPersec
bandwidth = objItem.CurrentBandwidth
result = FormatNumber(( bytesTotal / bandwidth) * 1000)
output = "Main NIC utilization : " & result & " % Utilized: " & objitem.BytesTotalPersec & " " & regValueDataMetric & " Total Bandwidth: " & bandwidth
WScript.Echo output


NEXT


If result >= 0.8 Then
Wscript.Echo "Utilizing Secondary NIC"
If regValueDataMetric = "1" Then
regValueDataMetric = "50"
Set WshShell = WScript.CreateObject("WScript.Shell")
WshShell.Run "netsh interface ipv4 set interface """"Wired"""" metric=" & regValueDataMetric, 0, TRUE
Set colItems = objWMIService.ExecQuery _
("Select * From Win32_NetworkAdapter Where NetConnectionID = '" & strMainNIC & "'")


End if
End If


If result <= 0.8 Then


If regValueDataMetric = "50" Then
regValueDataMetric = "1"


Set WshShell = WScript.CreateObject("WScript.Shell")
WshShell.Run "netsh interface ipv4 set interface """"Wired"""" metric=" & regValueDataMetric, 0, TRUE


Set colItems = objWMIService.ExecQuery _
("Select * From Win32_NetworkAdapter Where NetConnectionID = '" & strMainNIC & "'")


For Each objItem in colItems
strMACAddress = objItem.MACAddress
Wscript.Echo "MACAddress: " & strMACAddress
Next


Set colNetCard = objWMIService.ExecQuery _
("Select * From Win32_NetworkAdapterConfiguration where IPEnabled=TRUE" )


End if
End If


WScript.Sleep 1000


Next