Could you leave this as ``metrics`` rather than change the parameter name to ``metric_list``? That seems like an unnecessary interface change, and could break code where a user specified parameters as key, value pairs, for example:
```
single_file_beta(input_path='x.biom',metrics='unweighted_unifrac,weighted_unifrac')
```

Similarly, you have the flexibility here to allow ``metrics`` to be passed as a list or a string. Just do the following to make this change backward compatible:

```
try:
    metrics = metrics.split(',')
except AttributeError:
    pass
```