Nested Instance

See metric example from README:  ```rsnapshot_end_time{instance="{'instance': 'local.host'}"} 1.5909347717281592e+09```
The instance is set as a nested structure: `instance="{'instance': 'local.host'}"`

This also ends up in that way in prometheus with our tests. 
We think this is a bug or are we missing a special pushgateway configuration?