Tracing signals now include:
:parent {:keys [id uid instant]}
:root {:keys [id uid instant]}
The :instant info is undocumented, and useful for advanced cases like
the upcoming Span support for the OpenTelemetry handler.
Incl.:
1. Logger names are now used as namespaces.
- For SLF4J: these are typically class names.
- For tools.logging: these are typically *ns* strings.
2. These now have dedicated :kind (:slf4j, :tools.logging) to make it
easier for users to set kind-specific min levels.
Only downside/hesitation is that this info *must* be collected at the callsite,
which means that it affects the performance of *all* created signals.
Adds ~30-50 nsecs per signal.
