TrafficManager trafficManagerUserMetricsKeys API sends bad request for every endpoint in stable versions

https://docs.microsoft.com/en-us/rest/api/trafficmanager/traffic-manager-user-metrics-keys/get#code-try-0

In version 2018-04-01 support was added for the trafficManagerUserMetricsKeys child resource for Traffic Manager.

The two latest stable versions of this API do not work. I have tried via python and the above link.

The only parameter is the subscription ID so I can't see how this doesn't work.

Both result in:

```
{
"code": "BadRequest",
"message": "Operation input is malformed. Please retry the request."
}
```

I am also seeing the same error with the other endpoints, delete and create/update.

Start with the linked Traffic Manager user metrics keys API documentation and compare the two latest stable versions with the 2018-04-01 version mentioned in the report. Reproduce the get, delete, and create/update requests; done means the affected stable-version endpoints no longer return the reported BadRequest response.

Written by the indexing model from the issue text.