opentelemetry_oban: Add Attribute for Time Spent in Queue

[bradschwartz]

Is your feature request related to a problem? Please describe.

Something I'm interesting in keeping track of is how much time a job has spent waiting in the queue since being scheduled. I would be able to use this as an indicator to know if my workers are taking longer than typical to process jobs.

Describe the solution you'd like

I would just add a new attribute in the handle_job_start function here.

Something like:

diff --git a/instrumentation/opentelemetry_oban/lib/opentelemetry_oban/job_handler.ex b/instrumentation/opentelemetry_oban/lib/opentelemetry_oban/job_handler.ex
index 8d14f78..b31a989 100644
--- a/instrumentation/opentelemetry_oban/lib/opentelemetry_oban/job_handler.ex
+++ b/instrumentation/opentelemetry_oban/lib/opentelemetry_oban/job_handler.ex
@@ -70,7 +70,8 @@ defmodule OpentelemetryOban.JobHandler do
       :"oban.job.attempt" => attempt,
       :"oban.job.max_attempts" => max_attempts,
       :"oban.job.inserted_at" => DateTime.to_iso8601(inserted_at),
-      :"oban.job.scheduled_at" => DateTime.to_iso8601(scheduled_at)
+      :"oban.job.scheduled_at" => DateTime.to_iso8601(scheduled_at),
+      :"oban.job.queue_time_ms" => DateTime.diff(DateTime.utc_now(), scheduled_at, :millisecond)
     }
 
     span_name = "#{worker} process"

Describe alternatives you've considered
Continue to just calculate and add this attribute by hand manually.

Additional context
Add any other context or screenshots about the feature request here.

[xantrac]

The actual time spend in the queue should be given by the difference between the time when the job was scheduled_at and the time the job was attempted. Also we should probably only expose raw values and perform calculation in our Otel aggregator.

This PR should give us the data required to get such visbility

#511