message: Add an optional prefix param to ScopedStepContainer to avoid possible name clashes
diff: @@ -89,9 +89,17 @@ class ScopedStepContainer {
  // step_id: the unique ID of this step. Doesn't have to be sequential, just
  // has to be unique.
  // cleanup: callback to delete a container of this name.
  + // prefix: optional string prefix to disambiguate step containers.
  ScopedStepContainer(const int64 step_id,
  std::function<void(const string&)> cleanup)
  : name_(strings::StrCat("__per_step_", step_id)), cleanup_(cleanup) {}
  +
  + ScopedStepContainer(const int64 step_id,
  + std::function<void(const string&)> cleanup,
  + const string& prefix)
  + : name_(strings::StrCat("__", prefix, "_per_step_", step_id)),
  + cleanup_(cleanup) {}
  +
  ~ScopedStepContainer() { cleanup_(name_); }
  const string& name() const { return name_; }
  