Record the Delayed Job enqueue as a producer span

Enqueuing a Delayed Job now records an `enqueue.delayed_job` event on
the active transaction, so enqueues made from a web request or another
job show up in the event timeline. Delayed Job was the only background
job backend still missing this.

In collector mode that event also opens a producer span, matching the
other job backends and OpenTelemetry's own Delayed Job instrumentation.
Delayed Job has no envelope to carry trace context across the
enqueue/perform boundary, so -- like OpenTelemetry -- nothing is
injected and the producer and consumer spans are not linked.

Test it against Delayed Job's in-memory test backend under a new
`delayed_job` gemfile, driving the real enqueue and perform lifecycle in
both agent and collector mode.

Author: unflxw

.changesets/add-delayed-job-enqueue-instrumentation.md

@@ -0,0 +1,8 @@
+---
+bump: minor
+type: add
+---
+
+Instrument Delayed Job enqueues. Enqueuing a job now records an
+`enqueue.delayed_job` event on the active transaction, so enqueues made from
+within a web request or another job show up in the event timeline.