DOC: extract_metric: added doc
DOC: extract_metric: added doc

Former-commit-id: 7ab273a6cc46dc90bdc40585834e6d6c6144ad54
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox