- Allow to select reqwest backend using feature flags by @greaka.
- Add span fields to the serialized events by @chrismanning
- Change level to string mapping by @juumixx (each level is now differently
colored by Loki, previously: "debug", "informational", "notice", "warning",
"error"; now "trace", "debug", "info", "warn", "error" like the
tracinglevels.