About there is a property under the TelemetryService called "SupportedCollectionFunctions". This property looks like no order, but could we still follow below rule(this noted in spec) to implement this property? And if some other properties like this example, could we all implement by follow this rule?
It depends on whether or not you want to convey the max size of the array to the client in the GET response. In the "Array properties" clause, there is the following statement: "Services may pad an array property with null elements at the end of the sequence to indicate the array size to clients."
So, if you do want to convey the max size of the array to the client, then pad it with nulls. If you do not need to convey the max size of the array to the client, then simply return "SupportedCollectionFunctions": [ "Minimum" ].
That's correct; you would remove the rest of the array like with other arrays. The statement "A PATCH request with fewer elements than in the current array shall remove the remaining elements of the array." applies to all types of arrays.