add support for multiple outcomes to grid_performance.R

[era127]

The .estimate_metrics function does not support multiple outcomes and will produce an error. The easiest way to support it would be to change the data inside .esimate_metrics() to stack each outcome column into a tall table (similar to pivot_longer) and then continue with the code as it is now.

Is there interest in supporting this, or is there an alternative plan we could help implement?

Thanks