[metrics-bugs] #22279 [Metrics/metrics-lib]: simplify and avoid repetition in ParserHelper methods

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#22279: simplify and avoid repetition in ParserHelper methods
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     Reporter:  iwakeh               |      Owner:  metrics-team
         Type:  enhancement          |     Status:  new
     Priority:  Medium               |  Milestone:
    Component:  Metrics/metrics-lib  |    Version:
     Severity:  Normal               |   Keywords:  metrics-help
Actual Points:                       |  Parent ID:
       Points:                       |   Reviewer:
      Sponsor:                       |
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 Simplify and avoid repetition in ParserHelper methods and tests in
 ExtraInfoDescriptorTest accordingly. Cf. #22217 comments 6 an 7.  This
 includes a minor code clean-up and raising exceptions when duplicate keys
 are encountered.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22279>
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