If the created_at field is missing on a Sidekiq message (for example, if the message was created via Exq in Elixir), this library will raise undefined method '/' for nil and a stack that ends with:

 /.../vendor/bundle/ruby/3.3.0/gems/opentelemetry-instrumentation-sidekiq-0.26.1/lib/opentelemetry/instrumentation/sidekiq/middlewares/server/tracer_middleware.rb:78:in `time_from_timestamp'
       /.../vendor/bundle/ruby/3.3.0/gems/opentelemetry-instrumentation-sidekiq-0.26.1/lib/opentelemetry/instrumentation/sidekiq/middlewares/server/tracer_middleware.rb:34:in `call'
       /.../vendor/bundle/ruby/3.3.0/gems/sidekiq-7.1.6/lib/sidekiq/middleware/chain.rb:182:in `traverse'

Broken by this commit, as it tries to convert a nonexistent created_at

Simplified Reproduction:

1. With a Sidekiq server that's using opentelemetry-instrumentation-sidekiq
2. Add a Sidekiq-compatible job to Redis that doesn't include created_at via manual enqueue or Exq or some other method

_{Tip: React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding +1 or me too, to help us triage it. Learn more in our end user docs.}