If this field is empty, all custom and system-defined metric
descriptors are returned. Otherwise, the filter
</monitoring/api/v3/filters> specifies which metric
descriptors are to be returned. For example, the following
filter matches all custom metrics </monitoring/custom-
metrics>: :: metric.type =
starts_with("custom.googleapis.com/")
If this field is not empty then it must contain the
nextPageToken value returned by a previous call to this
method. Using this field causes the method to return
additional results from the previous method call.
