Metric should be just "gmail_label_unread", using a prometheus label for the gmail label (!)

As [available in this page](https://developers.google.com/gmail/api/reference/rest/v1/users.labels/list), I'd like this to become this:

```
gmail_Label_4301269950433810357_unread 2.0
```

to become some more like this:

```
gmail_label_unread{id="Label_4301269950433810357", name="spam"} 2.0
```