If you have your temperature monitoring working on an Arduino Leonardo, then the easiest solution is to use that. You will need to get the data to the server that runs your emonCMS.
If it is a local Raspberry Pi, then you can use a serial connection that is documented here:
Wiki:
https://wiki.openenergymonitor.org/index.php/EmonTx_V3.4#Direct_connection_emonTx_V3_.3E_Raspberry_Pi_GPIO
(Clearly, you use the Tx and Rx data pins that are in a different place on your Leonardo to the emonTx).
A little more information is here:
Wiring diagram: 4 CT emonBase using emonTx and Pi Zero W - #17 by pb66
If it is your emonPi, I understand you can use any GPIO pin for a second serial data input, but I am not a RPi expert so I cannot give you the details.
If the server running emonCMS is some distance away (or it is emoncms.org), then most likely the easiest solution is still to connect serially to a Raspberry Pi running emonCMS but send the data via your LAN or the Internet to the emonCMS where you have the dashboard.
If you have a WiFi LAN, then you can use the serial connection again, but this time to a Pi Zero W (which does not have an Ethernet socket, so you cannot use it with a wired LAN).