Expose some prometheus metrics

[yuvipanda]

To make operating this project easier, we should expose some prometheus metrics.

Turns out nfs-ganesha itself exposes prometheus metrics natively, but that is controlled by a compile time flag.

[yuvipanda]

I opened https://review.gerrithub.io/c/ffilz/nfs-ganesha/+/1204181 to try to change the default from OFF to ON, so we can rely on an upoming upstream build instead of having to compile stuff ourselves.

nfs-ganesha/nfs-ganesha#1038 is related.