Telemere signals are maps with {:keys [inst id ns level data msg_ ...]},
though they can be modified by call and/or handler transform (xfns).

Default signal keys:

`:schema` ------ Int version of signal schema (current: 1)
`:inst` -------- Platform instant [1] when signal was created, monotonicity depends on system clock
`:ns` ---------- ?str namespace of signal callsite
`:coords` ------ ?[line column] of signal callsite

`:kind` -------- Signal ?kind ∈ #{nil :event :error :log :trace :spy :slf4j :tools-logging <app-val> ...}
`:level` ------- Signal level ∈ #{<int> :trace :debug :info :warn :error :fatal :report ...}
`:id` ---------- Signal callsite ?id (usu. keyword) (common to all  signals created at callsite,              contrast with `:uid`)
`:uid` --------- Signal instance ?id (usu. string)  (unique to each signal  created at callsite when tracing, contrast with  `:id`)

`:msg_` -------- Arb app-level message  ?str              given to signal creator - may be a delay, always use `force` to unwrap!
`: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

`: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
`:ctx` --------- ?val of `*ctx*` (arb app-level state) when signal was created

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

`:sample` ------ Sample ?rate ∈ℝ[0,1] for combined call 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 transforms/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`