[metrics-bugs] #24434 [Metrics/Library]: Provide fallbacl list parser

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#24434: Provide fallbacl list parser
---------------------------------+--------------------------
     Reporter:  iwakeh           |      Owner:  metrics-team
         Type:  enhancement      |     Status:  new
     Priority:  Medium           |  Milestone:
    Component:  Metrics/Library  |    Version:
     Severity:  Normal           |   Keywords:
Actual Points:                   |  Parent ID:  #24429
       Points:                   |   Reviewer:
      Sponsor:                   |
---------------------------------+--------------------------
 Provide a parser interface and implementation for
 [https://gitweb.torproject.org/tor.git/tree/src/or/fallback_dirs.inc
 fallback mirror lists] (cf. parent ticket).

 Naming suggestions:
 * new interface: `o.tp.metrics.descriptor.FallbackMirrors` with possible
 sub-interface `FallbackMirrors.Mirror` or shorter `Fallbacks` and
 `Fallbacks.Mirror`
 * package for implementation: `o.tp.metrics.descriptor.fallback`

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24434>
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