The undertow attributes: "processing-time" and "max-processing-time" do not have "unit" defined. For instance, if you execute:
/subsystem=undertow/server=default-server/http-listener=default:read-resource-description
this will return:
...
"max-processing-time" =>

{ "type" => LONG, "description" => "The maximum processing time taken by a request on this listener", "expressions-allowed" => false, "nillable" => false, "access-type" => "metric", "storage" => "runtime" }

,
"no-request-timeout" =>

{ "type" => INT, "description" => "The length of time in milliseconds that the connection can be idle before it is closed by the container, defaults to 60000 (one minute)", "expressions-allowed" => true, "nillable" => true, "default" => 60000, "unit" => "MILLISECONDS", "access-type" => "read-write", "storage" => "configuration", "restart-required" => "no-services" }

,
"processing-time" => {
"type" => LONG,
"description" => "The total processing time of all requests handed by this listener",
"expressions-allowed" => false,
"nillable" => false,
"access-type" => "metric",
"storage" => "runtime"
...
According to the code:
https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/server/ConnectorStatisticsImpl.java#L63
attributes "processing-time" and "max-processing-time" should be shown as nano seconds.
I have just realised that only few attributes have "unit" defined - like "no-request-timeout" while the most of them is without it.