size_t bson_writer_get_length (bson_writer_t *writer);
Fetches the current length of the content written by the buffer (including the initial offset). This includes a partly written document currently being written.
This is useful if you want to check to see if you've passed a given memory boundry that cannot be sent in a packet. See bson_writer_rollback() to abort the current document being written.
The length of the underlying buffer.