[metrics-bugs] #22196 [Metrics/metrics-lib]: Configure descriptor sources using method chaining

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#22196: Configure descriptor sources using method chaining
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     Reporter:  karsten              |      Owner:  metrics-team
         Type:  enhancement          |     Status:  new
     Priority:  Medium               |  Milestone:
    Component:  Metrics/metrics-lib  |    Version:
     Severity:  Normal               |   Keywords:
Actual Points:                       |  Parent ID:
       Points:                       |   Reviewer:
      Sponsor:                       |
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 The two descriptor sources `DescriptorReader` and `DescriptorParser` are
 instantiated by `DescriptorSourceFactory`, configured, and then executed.
 This requires state checks and leads to quite verbose application code.
 An exception to this is `DescriptorCollector` which is state-less but
 which has a method with 5 parameters, which also does not produce very
 readable code.

 We should consider switching to configuring descriptor sources using
 method chanining.  [https://gitweb.torproject.org/user/karsten/metrics-
 lib.git/commit/?h=task-16225&id=f8c5862bbdfa0acefbbbd7d4899b997077c20196
 Here's a code sample] from a few months ago, written for another ticket.

 iwakeh rightly
 [https://trac.torproject.org/projects/tor/ticket/16225#comment:6 commented
 on that other ticket]: "configuration of a descriptor source by fluent-
 style (or method chaining) is fine, but metrics-lib currently has the
 DescriptorSourceFactory approach, which would need to be adapted. That is,
 I see two things: the ideas around the code of DescriptorSourceBuilder are
 ideas about a new way of configuration and not exception/error handling in
 metrics-lib, i.e. a different ticket (there is one for redesign of the
 interface hierarchy, I'll look for it). Second, the current configuration
 and descriptor source instanciation need to be considered together (in
 that other ticket)."

 Not assigning to a milestone on purpose.  This likely needs more thoughts
 before we can write code and make plans for releasing that.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22196>
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