Signal options (shared by all signal creators):

`:inst` -------- Platform instant [1] when signal was created, ∈ #{nil :auto <user-val>}
`: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          (common to all  signals created at callsite, contrast with `:uid`)
`:uid` --------- ?id of signal instance (unique to each signal  created at callsite, contrast with  `:id`)

`:msg` --------- Arb user-level ?message to incl. in signal: str or vec of strs to join (with `\space`)
`:data` -------- Arb user-level ?data    to incl. in signal: usu. a map
`:error` ------- Arb user-level ?error   to incl. in signal: platform error [2]

`:run` --------- ?form     to execute UNCONDITIONALLY; will incl. `:run-value` in signal
`:do` ---------- ?form     to execute   conditionally (iff signal allowed), before establishing `:let` ?binding
`:let` --------- ?bindings to establish conditionally (iff signal allowed), BEFORE evaluating `:data` and `:msg` (useful!)

`:ctx` --------- Custom ?val to override auto (dynamic `*ctx*`) in signal
`:parent` ------ Custom ?{:keys [id uid]} to override auto (dynamic) parent signal info in signal
`:location` ---- Custom ?{:keys [ns line column file]} to override auto signal creator callsite location

`:elidable?` --- Should signal be subject to compile-time elision? (Default: true)
`:sample-rate` - ?rate ∈ℝ[0,1] for signal sampling (0.75 => allow 75% of signals, nil => allow all)
`:when` -------- Arb ?form; when present, form must return truthy to allow signal
`:rate-limit` -- ?spec as given to `taoensso.telemere/rate-limiter`, see its docstring for details
`:middleware` -- Optional (fn [signal]) => ?modified-signal to apply when signal is created
`:trace?` ------ Should tracing be enabled for `:run` form?

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

handler-specific data that can just be ignored by other 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`