Metrics naming with overwrite args

[arielge]

When loading a metric with overwrite args (for example: metrics.char_edit_distance[reference_field=original_text]), this is not reflected in the result dict returned from the metric, which will have the original metric name. This creates a problem for example if one wants to call the same metric class twice, but with different overwrite args. In this case, the results dicts of the two metrics will have the same keys and they will overwrite each other.