Expose getBenchFn to be able to access benchmarks functions in a custom runner

[adriencaccia]

Clear and concise description of the problem

I am writing a custom runner for benchmarks. I used https://github.com/vitest-dev/vitest/tree/main/packages/vitest/src/runtime/runners/benchmark.ts as a reference and got blocked when I needed to access the registered functions through bench.
In NodeBenchmarkRunner's implementation, they are retrieved thanks to the following line

vitest/packages/vitest/src/runtime/runners/benchmark.ts

Line 61 in edad9b1

const benchmarkFn = getBenchFn(benchmark)

However, getBenchFn is not exposed, so I cannot access it in my custom runner.

Suggested solution

Export getBenchFn from a new vitest/bench namespace, or somewhere else.

Alternative

No response

Additional context

The equivalent getFn function to retrieve test functions is already exported from vitest/suite:

vitest/packages/runner/src/index.ts

Line 4 in bdc06dc

export { setFn, getFn } from './map'

So I think that it would make sense to export getBenchFn, and maybe getBenchOptions too.

I would be more than happy to submit a pull request if you accept this proposal!

For context, the custom runner would be to support vitest on https://codspeed.io/, I already talked about it on discord with @Aslemammad

P.S. Thank you for such an amazing project!

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