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

Default signal 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 :slf4j :tools-logging <app-val> ...}
`:id` ---------- ?id of signal          (common to all  signals created at callsite,              contrast with `:uid`)
`:uid` --------- ?id of signal instance (unique to each signal  created at callsite when tracing, contrast with  `:id`)

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

`:run-form` ---- Unevaluated ?form given to signal creator 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 app-level state) when signal was created
`:parent` ------ ?{:keys [id uid]} of parent signal, present in nested signals when tracing
`:root` -------- ?{:keys [id uid]} of root   signal, present in nested signals when tracing

`:location` ---- ?{:keys [ns file line column]} signal creator callsite
`:ns` ---------- ?str namespace of signal creator callsite, same as (:ns     location)
`:line` -------- ?int line      of signal creator callsite, same as (:line   location)
`:column` ------ ?int column    of signal creator callsite, same as (:column location)
`:file` -------- ?str filename  of signal creator callsite, same as (:file   location)

`:host` -------- (Clj only) {:keys [name ip]}       info for network host
`:thread` ------ (Clj only) {:keys [name id group]} info for thread that created signal

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

<kvs> ---------- Other arb app-level ?kvs given to signal creator. Typically NOT included
                 in handler output, so a great way to provide custom data/opts for use
                 (only) by custom middleware/handlers.

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`