Improvements to CompareMetrics

[kachulis]

Adds the following additional features to CompareMetrics:

1. Command line parameter METRICS_NOT_REQUIRED to specify metrics which may be missing from either input.
2. Command line parameter KEY to specify a metric column to use as a key for metrics comparisons. This is useful for checking if the only difference between two metrics files is the order of the rows.
3. Option to output a tsv table of metrics differences. This is much easier to evaluate than the current text output.

[meganshand]

👍

[nikellepetrillo]

This looks good to me, thank you!