Allow DelayedJob to be extended and JOB_CLASS_REGEXP redefined

[tekkamanendless]

In our testing and use of this tool, we found that the value of
JOB_CLASS_REGEXP for the DelayedJob instrumentation didn't match
what our values looked like.

The original code is looking for something like this:

job_class: MyClass

However, in our environment, our values look something more like
this (YAML data with no "job_class" to be seen):

--- !ruby/struct:MyJob
endpoint_id:
payload:

Anyway, we wanted to just change the JOB_CLASS_REGEXP value by subclassing
DelayedJob and registering that plugin, but it didn't work because even
the subclass would use the original class's value of JOB_CLASS_REGEXP.

This update switches the use of JOB_CLASS_REGEXP to allow for a subclass
to override the value using self.class::JOB_CLASS_REGEXP.

Our desired workflow is to allow for this:

class MyDelayedJob < PrometheusExporter::Instrumentation::DelayedJob
  JOB_CLASS_REGEXP = %r{ruby/(?:object|struct):\s*(\w+)}.freeze
end
MyDelayedJob.register_plugin