It'll be nice if we can store the creation date of each MongoS in the mongos collections.

mongos> db.getSiblingDB("config").mongos.find().pretty()
{
	"_id" : "hostname.local:27017",
	"advisoryHostFQDNs" : [
		"hostname.local"
	],
	"mongoVersion" : "4.4.5",
        "created": ISODate("2021-05-31T11:42:24.629Z"),
	"ping" : ISODate("2021-05-31T11:42:24.629Z"),
	"up" : NumberLong(430953),
	"waiting" : true
}

This could just be the time when the mongos document is created.

Sometimes, this kind of info help Devops or DBAs investigate environmental issues etc, with the CSRS and shards, we can get the creation date of the replica set from the replica set ID, but we don't have a way to tell MongoS creation date reliably.