[tor-bugs] #19640 [Metrics/metrics-lib]: review and improve interface hierarchy

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#19640: review and improve interface hierarchy
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 Reporter:  iwakeh               |          Owner:  karsten
     Type:  defect               |         Status:  new
 Priority:  Medium               |      Milestone:  metrics-lib 1.5.0
Component:  Metrics/metrics-lib  |        Version:
 Severity:  Normal               |     Resolution:
 Keywords:                       |  Actual Points:
Parent ID:                       |         Points:
 Reviewer:                       |        Sponsor:
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Comment (by iwakeh):

 `ParseHelper` should be split into a utility class that is part of the
 interface and the implementation specific part inside `impl`.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19640#comment:4>
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