Pretty-printed (multi-line + indentation) JSON is easier to read, however, we should not do it in our mostly-structured logs, because:

1) it makes it harder to parse with automatic tools
2) multiple lines can intertwine with other process output making it harder to read or extract manually
3) pretty-printing can be done with additional tools, for instance, Parsley now supports pretty-printing JSON in 'bookmarked' lines in 'resmoke' mode

This change must be activated by an opt-in parameter (see SERVER-97956) in order not to break the upstream dependencies (e.g., build-baron tools).