Add ability to pass total print time in seconds via gcode placeholder

[markusastone]

Hello!

I'd like the ability to have a placeholder in the gcode than can be replaced with the total print time in seconds, so that this can be used for macros etc in Klipper.

One such use case is for print farm automation, where the heated bed can be turned off ~5 minutes before a print finishes, so that the bed can begin cooling to release the part whilst the part is being finished, thus saving on 'downtime' between prints, waiting for the bed to cool before part ejection.

Presently, there is no way within Klipper to natively grab the total print time of a file within gcode, and I believe one of the best ways would be via this post-processing script.

Alternatively, if we can have a means of getting the estimated print time output in any other means that isn't the file's name, or a ; comment, would be greatly appreciated.

Cheers!