Step-by-step guide
Since Metview version 5.26.0, when installed on ECMWF's HPC under the ecmwf-toolbox module, its MARS Retrieval icon is dynamically configured at startup time to use a given version of the command-line MARS client along with its particular set of MARS definitions. By default, this will be the same as the bare 'mars' command, but it can also be modified by setting the environment variable METVIEW_MARS_COMMAND. In order to run the 'mtg2' version of the MARS client, you would run 'mars -g', so in order to tell Metview to use this version of the client, you would start Metview like this:
METVIEW_MARS_COMMAND="mars -g" metview &
If running Metview from a Python environment, ensure to set this environment before your 'import metview', either from within the Python session, or from the shell before starting Python.