feat: tracing logs in estimator

[jakmeier]

Allow to select tracing levels and targets with the RUST_LOG environment variable when the estimator is running without the --tracing-span-tree flag.

[matklad]

Why do we need to check for --debug flag though? Seems fine to just do this unconditionally?

[jakmeier]

Why do we need to check for --debug flag though? Seems fine to just do this unconditionally?

I was thinking about interactions with tracing_span_tree, so we can enable them individually or combined. But no strong opinion, if you think it is fine, I can also enable this unconditionally.

[matklad]

Hm, would it work to enable them combined? I think the two would conflict. In this sense, I'd probably expcet

if span_tree {
  enable-span-tree
} else {
  enable-normal-logs
}

[jakmeier]

Yeah that makes sense!