# Returns the average value of the numeric values
	#
	# https://docs.mongodb.com/manual/reference/operator/aggregation/avg/
	#
	# ~~~json
	# { $avg: <expression> }
	# ~~~
	#
	# `$avg` ignores non-numeric values.
	fun avg(field: String, expression: Serializable): MongoGroup do
		return acc("avg", field, expression)
	end