Default metrics buckets for kamon-system-metrics are too small

[jatcwang]

https://github.com/kamon-io/Kamon/blob/7f806223406e9d9a8cb57334975f80afa438ad1c/reporters/kamon-prometheus/src/main/resources/reference.conf

The default buckets sizes are too small to provide useful information since all the collected values are put into the largest bucket which is currently any value > 1MB (e.g. jvm.memory.used)

I think a good way to address this is to add per-metric override in kamon-system-metrics' reference.conf.
If you're happy with this solution I'll raise a PR :) Thanks

[ivantopo]

Hey @jatcwang, that would be lovely! This is something I wanted to do for quite some time but just didn't find the time to do so. That PR would be more than welcome 🙏

[ivantopo]

Just one note: probably it makes more sense to add those settings on the kamon-prometheus' reference conf instead of system metrics