Specify metrics in threshold_perf()

[szego]

Description

So that the user can specify which metrics are computed when calling threshold_perf(), we add a new metrics argument that accepts a yardstick metric set.

By default this argument is NULL, in which case the function computes the default metrics. If not NULL, the function checks that it's an appropriate metric set (class metrics only) and computes only the metrics provided in the set.

Motivation and Context

Fixes #25

How Has This Been Tested?

Tested using yardstick 0.0.7 on R 4.0.2.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Note that I did not add an example in the docs of threshold_perf() that uses the new feature.

[szego]

Forgot to note that the code that verifies the passed metric set (lines 106 to 117 in threshold_perf.R) incorporates some code from tune::check_metrics(). See lines 305 to 310 here: https://github.com/tidymodels/tune/blob/a97576f3f3e36f362388de7e2f3ef2df8ab3a38f/R/checks.R#L305

[topepo]

Sorry for the long delay on this.

Since we were working on similar problems related to calibration I made a separate branch and PR to solve this (but credit you in the news file).

Thanks for getting this going!

[github-actions]

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