Reduce the number of custom Dataflow metrics (`Your project already contains 100 Dataflow-created metric descriptors`).

```
Your project already contains 100 Dataflow-created metric descriptors,
so new user metrics of the form custom.googleapis.com/* will not be created.
However, all user metrics are also available in the metric
dataflow.googleapis.com/job/user_counter. If you rely on the custom metrics,
you can delete old / unused metric descriptors.
See https://developers.google.com/apis-explorer/#p/monitoring/v3/monitoring.projects.metricDescriptors.list
and https://developers.google.com/apis-explorer/#p/monitoring/v3/monitoring.projects.metricDescriptors.delete
```