message: Add missing member function span::subspan
diff: @@ -168,6 +168,10 @@ public:
  return {begin_ + offset, num_bytes};
  }
  + constexpr span subspan(size_t offset) const {
  + return {begin_ + offset, size_ - offset};
  + }
  +
  constexpr span first(size_t num_bytes) const {
  return {begin_, num_bytes};
  }
  