Normally it is possible in jupyter or iPython notebooks to show lines number for a cell, however I don't see where in Google Colaboratory (Colab).
Yep, the shortcut (Ctrl + M + L) works,
other option is use the bar menu, at tool -> preference -> show line numbers
Update: new path: Tools -> Settings -> Editor -> show line numbers
Holding Ctrl and pressing ML (one by one) switches on/off line numbers in the cells containing code.
I see it under Tools->Settings->Editor (as on On 1//1/2020)