Add system stats to telemetry

[Mirko-von-Leipzig]

We probably want to track at least the following:

• storage disk space used and available
• memory use and remaining
• cpu usage and info

Ideal would also be to list our own process's usage specifically, including

• database size
• database WAL size
• block storage used
• memory used
• cpu times etc

Remember to check out the open-telemetry naming specification for system stats (yes, the page is about metrics, but we're injecting metrics into wide-events so let's stick with the conventions).

Implementation

Two options I can think of:

1. Background thread that periodically posts these stats as an event
2. Inject them into every span we emit

I'm assuming getting this information requires syscalls, and as such its not reasonable to do (2) in the actual span processor. Instead we likely still want a background thread to periodically update the values.

https://docs.rs/sysinfo/latest/sysinfo/ is a popular option I found, but maybe do some more research first. We want something low-cost. I also see that macOs could be problematic due to security permissions - we'll have to check that out as well.