Various agg stages are subject to internal limits, defined as setParameters in query_knobs.idl. Alex Bevilacqua would find it useful if these values were reported in agg explain. Since these are static settings rather than runtime stats, I think they would be reported at any explain verbosity level ("queryPlanner", "executionStats", or "allPlansExecution").
In particular, exposing at least following would be useful:
Before scheduling this ticket, we should choose the exact set of knobs we want to expose in explain, and we should choose the names with which we want to expose them.