We've recently done several work items in order to make interesting query system related event counters available in serverStatus() and FTDC:

• SERVER-63642: Metrics related to the classic and SBE multi-planners
• SERVER-40382: Plan cache memory consumption
• SERVER-71529: Metrics for plan cache size
• SERVER-70025: Plan cache hits and misses

One related metric that we don't have yet is a counter for the number of times that plan cache replanning occurs. This would be interesting both in field/diagnostics contexts, since high replanning rates may be associated with performance problems that could be worked around with features like hints or persistent query settings (see SERVER-79039). It would also be interesting for query product management to be able to study the prevalence of replanning events. This could help us understand how to improve or evolve the product's behavior with respect to replanning.