message: calls_x2 in StackFrame
  Summary:
  Store how many time function was called.
  Used in the following diff to initialize heap summary from heap allocation stacks.
diff: @@ -375,6 +375,9 @@ mod flame {
  /// Time spent in this frame excluding callees.
  /// Double, because enter/exit are recorded twice, in drop and non-drop heaps.
  time_x2: SmallDuration,
  + /// How many times this function was called (with this stack).
  + /// Double.
  + calls_x2: u32,
  }
  #[derive(Clone, Dupe)]
  @@ -386,6 +389,7 @@ mod flame {
  callees: Default::default(),
  allocs: Default::default(),
  time_x2: SmallDuration::default(),
  + calls_x2: 0,
  })))
  }
  @@ -460,6 +464,7 @@ mod flame {
  if let Some(last_time) = self.last_time {
  self.current.last_mut().unwrap().0.borrow_mut().time_x2 +=
  time.saturating_duration_since(last_time);
  + self.current.last_mut().unwrap().0.borrow_mut().calls_x2 += 1;
  }
  let frame = match self.current.last() {
  