"Spy" signal creator, emphasizing form + level.

API: [form] [level-or-opts form] => form's result (value/throw) (unconditional)
Default kind:  `:spy`
Default level: `:info`

When filtering conditions are met [4], creates a Telemere signal [3] and
dispatches it to registered handlers for processing (e.g. writing to
console/file/queue/db, etc.).

Examples:

  (spy! (+ 1 2))         ; %> {:kind :trace, :level :info, :run-form '(+ 1 2),
                         ;     :run-val 3, :run-nsecs <int>, :parent {:keys [id uid]}
                         ;     :msg "(+ 1 2) => 3" ...}
  (spy! ::my-id (+ 1 2)) ; %> {... :id ::my-id ...}
  (spy!
    {:let  [x "x"] ; Available to `:data` and `:msg`
     :data {:x x}}

    (+ 1 2)) ; %> {... :data {x "x"}, :msg_ "My msg: x" ...}

Tips:

  - Test using `with-signal`: (with-signal (spy! ...)).
  - Supports the same options [2] as other signals [1].

  - Identical to `trace!`, but emphasizes form + level rather than form + id.

  - Useful for debugging/monitoring forms, and tracing (nested) execution flow.
  - Execution of `form` arg may create additional (nested) signals.
    Each signal's `:parent` key will indicate its immediate parent.

  - Can be useful to wrap with `catch->error!`:
      (catch->error! ::error-id (spy! ...)).

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[1] See `help:signal-creators` - (`signal!`, `log!`, `event!`, ...)
[2] See `help:signal-options`  - {:keys [kind level id data ...]}
[3] See `help:signal-content`  - {:keys [kind level id data ...]}
[4] See `help:signal-flow`     - (filters, handling, etc.)