public <T> AsyncMutateInBuilder arrayAppend(String path, T value) {
        this.mutationSpecs.add(new MutationSpec(Mutation.ARRAY_PUSH_LAST, path, value));
        return this;
    }
public < T > AsyncMutateInBuilder arrayAppend ( String path , T value ) { this . mutationSpecs . add ( new MutationSpec ( Mutation . ARRAY_PUSH_LAST , path , value ) ) ; return this ; }
Append to an existing array, pushing the value to the back/last position in
the array.

@param path the path of the array.
@param value the value to insert at the back of the array.