message: add init_classes metric [easy]
  Summary: This is a behavior preserving change.
diff: @@ -71,6 +71,8 @@ class RemoveUnusedFields final {
  return m_unremovable_unread_field_puts;
  }
  + size_t init_classes() const { return m_init_classes; }
  +
  private:
  bool is_blocklisted(const DexField* field) const {
  return m_config.blocklist_types.count(field->get_type()) != 0 ||
  @@ -210,6 +212,7 @@ class RemoveUnusedFields final {
  field->get_class());
  if (init_class_insn) {
  new_insns.push_back(init_class_insn);
  + m_init_classes++;
  }
  }
  if (replace_insn) {
  @@ -240,6 +243,7 @@ class RemoveUnusedFields final {
  std::unordered_set<const DexField*> m_vestigial_objects_written_fields;
  field_op_tracker::TypeLifetimes m_type_lifetimes;
  std::atomic<size_t> m_unremovable_unread_field_puts{0};
  + std::atomic<size_t> m_init_classes{0};
  };
  } // namespace
  @@ -262,6 +266,7 @@ void PassImpl::run_pass(DexStoresVector& stores,
  rmuf.vestigial_objects_written_fields().size());
  mgr.set_metric("unremovable_unread_field_puts",
  rmuf.unremovable_unread_field_puts());
  + mgr.set_metric("init_classes", rmuf.init_classes());
  if (m_export_removed) {
  std::vector<const DexField*> removed_fields(rmuf.unread_fields().begin(),
  