Fix naming bug for `gitlab_model` and `vertex_gecko`

This MR fixes a bug found in the results/v2 data collection that named the field that records the length of the prompt result as length instead of result_length. The script to do this leveraged one of the injection scripts previously merged and thus has not been included in the MR.

Also in the MR is renaming all locations where length is introduced as one of the fields in the model response.

cc @jayswain