[Feature]: Allow setting a custom coverage reporter

[benjaminjkraft]

🚀 Feature Proposal

The config option collectCoverage enables a bunch of things throughout Jest. One of these is adding the built-in CoverageReporter to the list of reporters. It would be nice if there were a way to override this, and instead add a custom reporter(s) that makes use of the same data but reports it in other ways.

Motivation

My specific motivation is to experiment with a reporter that merges coverage periodically throughout the test run, rather than only at the end, to work around #14287. In principle, everything I need to change is accessible from the existing reporter extension point. But there's no way to enable coverage collection without the default coverage reporter.

In general, this seems like it could be useful for reporting the coverage in a different format -- for example perhaps we want to use some coverage technique not supported by Istanbul.

Example

(see specific motivation)

Pitch

This seems like a way to allow improvements outside of jest with minimal new API surface. The coverage reporter is already complicated enough; right now something like #14287 can only be solved in jest itself. The API surface all already exists; one could already use the same APIs to add an additional coverage reporter. So this seems net-valuable.

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