feat(coverage): autoUpdate to support percentage formatting

[Battjmo]

close #8424

Description

I've found that when I have the boolean autoUpdate flag enabled and run coverage reports on the same code multiple times, the coverage amounts vary but up to 0.03%. This means code will arbitrarily fail to pass the coverage requirements. Also I don't really care about my coverage percentage being accurate down to two decimal places, which is how V8 reports them.

To address this, I've update the autoUpdate flag to accept either a boolean or a function. If it's a boolean it will keep working as it did before. If it's a function, that function will run on each coverage percentage returned by the engine and return the result, which should be a number or undefined.

So now you can do something like...

// in your vitest.config.mjs
{
"coverage": {
  "thresholds": {
      thresholds: {
        statements: 94,
        branches: 90,
        lines: 94,
        functions: 90,
        autoUpdate: (value) => {
          if (value) {
            // do something to format the value
            return Math.floor(value);
          }
        },
      },
}
}
}

Now when you run test:coverage or whatever your coverage script is, your coverage thresholds will come back rounded down to the nearest whole number.

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[AriPerkkio]

Looks good, thanks for the feature! 💯

[Battjmo]

Thanks @AriPerkkio!