message: Write unused capacity into flamegraph output
  Summary:
  People ask why profiler shows less memory when `buck2` process uses.
  Here is the answer.
  Following diff adds unused capacity to CSV.
diff: @@ -384,6 +384,9 @@ impl AggregateHeapProfileInfo {
  pub fn gen_flame_graph(&self) -> String {
  let mut data = FlameGraphData::default();
  self.root().write_flame_graph(data.root());
  + data.root()
  + .child(ArcStr::new_static("unused_capacity"))
  + .add(self.unused_capacity.get() as u64);
  data.write()
  }
  