[tor-bugs] #20039 [Metrics/metrics-lib]: integrate `DescriptorIndexCollector` in a fully backward-compatible way

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#20039: integrate `DescriptorIndexCollector` in a fully backward-compatible way
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 Reporter:  iwakeh               |          Owner:  iwakeh
     Type:  enhancement          |         Status:  assigned
 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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Changes (by iwakeh):

 * status:  new => assigned
 * owner:  karsten => iwakeh


Comment:

 The index.json functionality belongs to CollecTor; metrics-lib only keeps
 a minimal copy of the *Node classes to facilitate reading of index.json
 variants.

 The //master// copy is located in CollecTor incl. the relevant tests.

 Reasoning:
 Best would be to issue an independent module of CollecTor to provide
 index.json* reading functionality. In order to reduce api maintenance this
 is not done and a simple copy is kept in metrics-lib to enable index.json*
 reading.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20039#comment:1>
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