If this field is empty, all custom and system-defined metric
descriptors are returned. Otherwise, the filter
<https://cloud.google.com/monitoring/api/v3/filters>
specifies which metric descriptors are to be returned. For
example, the following filter matches all custom metrics
<https://cloud.google.com/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.
