message: Add convenience function to consumer resources
diff: @@ -386,6 +386,11 @@ public:
  }
  }
  + template <class Coordinator>
  + auto observe_on(Coordinator* ctx) {
  + return ctx->make_observable().from_resource(*this);
  + }
  +
  explicit operator bool() const noexcept {
  return ctrl_ != nullptr;
  }
  