[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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Comment (by iwakeh):

 Second change:

 `DescriptorIndexCollector#collectDescriptors` will take either a full URL
 to an index.json variant or in case a base URL like
 {{{https://x.y.z/a/b}}} is given attempt to download index.json.gz from
 there, i.e. attempt to download {{{https://x.y.z/a/b/index.json.gz}}}.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20039#comment:2>
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