Signals are initially maps with {:keys [inst id ns level data msg_ ...]},
though they can be modified by call and/or handler middleware.

Default keys:

`:schema` - Int version of signal schema (current: 1)
`:inst`   - Platform instant [1] when signal was created
`:level`  - Signal level ∈ #{<int> :trace :debug :info :warn :error :fatal :report ...}
`:kind`   - Signal ?kind ∈ #{nil :event :error :log :trace :spy <user-val> ...}
`:id`     - ?id of signal call     (common to all signals created by signal call, contrast with `:uid`)
`:uid`    - ?id of signal instance (unique to each signal created by signal call, contrast with `:id`)

`:msg`   - Arb user-level message  ?str              given to signal call
`:data`  - Arb user-level data     ?val (usu. a map) given to signal call
`:error` - Arb user-level platform ?error [2]        given to signal call

`:run-form` - Unevaluated ?form given to signal call as `:run`
`:run-val`  - Successful return ?val of  `:run` ?form
`:run-nsecs`- ?int nanosecs runtime of   `:run` ?form
`:end-inst` - Platform ?instant [1] when `:run` ?form completed

`:ctx`      - ?val of `*ctx*` (arb user-level state) when signal was created
`:parent`   - ?{:keys [id uid]} of parent signal, present in nested signals when tracing
`:location` - ?{:keys [ns file line column]} signal call location
`:ns`       - ?str namespace of signal call, same as (:ns     location)
`:line`     - ?int line      of signal call, same as (:line   location)
`:column`   - ?int column    of signal call, same as (:column location)
`:file`     - ?str filename  of signal call, same as (:file   location)

`:sample-rate` - ?rate ∈ℝ[0,1] for combined call AND handler sampling (0.75 => allow 75% of signals, nil => allow all)

<extra-kvs> - Arb user-level ?kvs given to signal call

If anything is unclear, please ping me (@ptaoussanis) so that I can improve these docs!

[1] `java.time.Instant`   or `js/Date`
[2] `java.lang.Throwable` or `js/Error`