Printing method for metrics

[EmilHvitfeldt]

• I think a print method for the metric functions, if people are going to be creating them and dealing with them:

yardstick::sens
#> function (data, ...) 
#> {
#>     UseMethod("sens")
#> }
#> <bytecode: 0x117ee7e30>
#> <environment: namespace:yardstick>
#> attr(,"direction")
#> [1] "maximize"
#> attr(,"class")
#> [1] "class_metric" "metric"       "function"

## just an idea:
format.metric <- function(x, ...) {
    first_class <- class(x)[[1]]
    cli::cli_format_method({
        cli::cli_h3("A {.cls {first_class}} function to {attr(x, 'direction')} metrics")
    })
}

print.metric <- function(x, ...) {
    cat(format(x), sep = "\n")
    invisible(x)
}

yardstick::sens
#> 
#> ── A <class_metric> function to maximize metrics

^{Created on 2023-05-15 with reprex v2.0.2}

Originally posted by @juliasilge in #434 (review)

[simonpcouch]

With an eye for #434, would also be awesome if it included something like:

if (!is.null(atrr(x, "by")) {
  cat(cli::cli_text("Internally grouped and summarized by {.field by}."))
}

[github-actions]

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