Add log level override via RUST_LOG environment variable

[XA21X]

For ad-hoc testing and when writing tests, it is convenient to be able to override the log level without changing code or config. This is a common use case that will hopefully be supported by upstream in the future - see tokio-rs/tracing#1466.

Example usage:

RUST_LOG=debug,shotover-proxy=trace shotover-proxy [...]

[rukai]

Thanks for splitting it out!
This change makes sense to me.

Just a few requests.

[rukai]

It seems try_new error message is worse than new.
We now only get Error: invalid filter directive
But we used to get: ignoring `blah=blah`: invalid filter directive

We can improve it with:

EnvFilter::try_new(&directives).map_err(|x| anyhow!("{}: {}", x, directives))?

We can then further improve it by using the add_directive API which lets us fail on the directives individually and then we can report which directive source specifically caused the issue.
https://docs.rs/tracing-subscriber/0.2.20/tracing_subscriber/filter/struct.EnvFilter.html#method.add_directive
Feel free to do just the first solution or both solutions, up to you.

[rukai]

Wow, a unittest as well!

[XA21X]

Pending second review @benbromhead?, thanks