update tracing log format, remove unnessary rustwide log fields

[syphar]

• our internal Pretty format used the tracing-subscriber Full format, which is quite noisy.
• I now added support for all log-formats that tracing-subscriber supports by default.
• for local cli usage, compact will be the default, which puts the msg/span fields at the end
• the default for the log-format is now determined by is_terminal or not. So we can drop the explicit setting on our server, and will have nicers logs in docs_rs_admin, or other CLI calls.
• rustwide build logs came via tracing_log::LogTracer, which appended 4 additional fields to each line. We don't need most of these, so I added a custom adapter that will just emit the log-message and target to tracing, and drop the rest. This only affects tests, local CLI / dev usage, etc. On our production build-servers, the log-tracer was disabled anyways, since we don't emit the build logs, just capture and upload them. But: this will be much more interesting when the build-cli for CI makes progress, so the output is actually usable.

next steps after this

• reduce the level from INFO to DEBUG for some spans (for example internal function calls in rustwide)
• remove all fields from spans (via instrument(skip_all)), only add back what we really want to see in the logs, and never use the debug representation of structs in logs.
• move some fields from spans to events, so they get logged once, and not again in all child-method-calls.

tracing log format

With an active build{crate=example} span, the previous full formatter
repeated the span fields before each bridged message:

2026-09-03T12:00:00Z  INFO build{crate=example}: log: Compiling example

The compact formatter keeps the span name before the message and appends
the span fields after it:

2026-09-03T12:00:00Z  INFO build: log: Compiling example crate=example

[syphar]

Damn, some compile errors / test errors.

Will fix them tomorrow or later or next week, then ping you again

[syphar]

r? @GuillaumeGomez