refactor: make Prometheus functionality optional

[DorianNiemiecSVRJS]

I have found that the prometheus crate depends on versions of the pathbuf crate that are vulnerable to RUSTSEC-2024-0437. To mitigate this risk and reduce software bloat, I propose making the ocsp-stapler crate optionally depend on the prometheus crate.

I have added a feature flag for prometheus, which is enabled by default to ensure that existing users continue to have access to its functionality without disruption. Users who do not require prometheus can disable this feature in their configuration.

This change has been tested to ensure that it does not break existing functionality, and I updated the documentation to reflect this new feature flag.

[blind-oracle]

Thanks, makes sense!

[DorianNiemiecSVRJS]

Thank you for merging!
I would like to see a new version of the ocsp-stapler crate being released, just like after my previous pull request. I am using this crate in a web server that I am building, and would like to switch back to a crate from crates.io registry instead of my fork.

[blind-oracle]

I will release a new version asap