## Problem
We want to switch proxy and ideally all Rust services to structured JSON
logging to support better filtering and cross-referencing with tracing.
## Summary of changes
* Introduce a custom tracing-subscriber to write the JSON. In a first
attempt a customized tracing::fmt::FmtSubscriber was used, but it's very
inefficient and can still generate invalid JSON. It's also doesn't allow
us to add important fields to the root object.
* Make this opt in: the `LOGFMT` env var can be set to `"json"` to
enable to new logger at startup.
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