class_docstr: This class wraps allocation tracing in a way that pretends to be a
  clock, that is, one can ask for "now" snapshots, and then
  difference them to work out the "elapsed" allocations (that is,
  the total size difference) between them.
  
  The __sub__ (-) operator counts both allocations (positive) and
  frees (negative) between the two "MallocInstant"s, and so gives
  the delta in the size of the allocations, not the total size of
  new allocations.
code: def __init__(self, snapshot: tracemalloc.Snapshot):
      self._snapshot = snapshot
  
file_path: stellargraph/tests/test_utils/alloc.py
imports: import gc
  import pytest
  import tracemalloc
label_desc: Use image node_id 1 to create a new MallocInstant object  with example: obj = MallocInstant(snapshot)