Finding the heap low water mark when using nRFConnect/Zephyr

I need to monitor how much heap was ever left available, as a minimum, after I have run my CI test suite.  This is the true minimum heap ever remaining, so taking account of calls to malloc() as well as to Zephyr's memory management APIs, i.e. the true underlying heap low water mark.

I could do this on NRF52 as I had the implementation of _sbrk() that FreeRTOS called into (the Dave Nadler one).

What is the equivalent in the nRFConnect/Zephyr world on NRF53?