Start polls runtime.ReadMemStats with interval d and updates the package level variables.

Alloc is the number of bytes allocated and not yet freed by the application.

NumGC is the number of collections.

PauseNS is the total number of nanoseconds the GC has paused the application.

TotalAlloc is the total number of bytes allocated by the application.

Var is an atomic variable satisfying expvar.Var.