Logging SFP Optic Signal Strength

I’m trying to create a script to log an optics signal strength, but its just not working. When I run it, it outputs the whole status of the sfp port, instead of just the tx and rx power. Here is my command:

/interface ethernet monitor sfp-sfpplus1 once do={
:log info ("sig:Rx/Tx:$"signal-strength"$"tx-signal-strength")
}

I can’t get it to actually only grab the tx and rx power info.