Capture the async call graph for the current task or the provided Future.
The graph is represented with three data structures:
* FutureCallGraph(future, call_stack, awaited_by)
Where 'future' is an instance of asyncio.Future or asyncio.Task.
'call_stack' is a tuple of FrameGraphEntry objects.
'awaited_by' is a tuple of FutureCallGraph objects.
* FrameCallGraphEntry(frame)
Where 'frame' is a frame object of a regular Python function
in the call stack.
Receives an optional 'future' argument. If not passed,
the current task will be used. If there's no current task, the function
returns None.
If "capture_call_graph()" is introspecting *the current task*, the
optional keyword-only 'depth' argument can be used to skip the specified
number of frames from top of the stack.
If the optional keyword-only 'limit' argument is provided, each call
stack in the resulting graph is truncated to include at most
``abs(limit)`` entries. If 'limit' is positive, the entries left are
the closest to the invocation point. If 'limit' is negative, the
topmost entries are left. If 'limit' is omitted or None, all entries
are present. If 'limit' is 0, the call stack is not captured at all,
only "awaited by" information is present.