possibiliy to reduce bundle size by 40kb about nuxt-prometheus

[artmizu]

@pi0 thanks for the request

but the purpose of prom-client in this module is not only about creating an export file with rendering time metrics and API requests. It's also to create a complete solution for a wide set of standard NodeJS metrics: about memory consumption, CPU and other common things.

So users can integrate this module into their projects and not have to worry about integrating additional modules

[pi0]

I see good point! Indeed as mentioned it is mostly useful optimization for Nodeless targets. It seems harder than i thought this way. Moved issue to unjs/unenv#70 to not forget. If we find a way for unenv mock, it will seamlessly be added for Nuxt projects and your module. Cheers.