[metrics-bugs] #25700 [Metrics/Onionoo]: Fetch descriptors from both CollecTor instances

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#25700: Fetch descriptors from both CollecTor instances
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     Reporter:  karsten          |      Owner:  karsten
         Type:  enhancement      |     Status:  assigned
     Priority:  Medium           |  Milestone:  Onionoo 1.12.0
    Component:  Metrics/Onionoo  |    Version:
     Severity:  Normal           |   Keywords:
Actual Points:                   |  Parent ID:
       Points:                   |   Reviewer:
      Sponsor:                   |
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 As discussed at the last team meeting, we should attempt to make Onionoo
 more robust by fetching descriptors from both CollecTor instances.

 I started working on this by extending `DescriptorSource` and am currently
 trying it out. As soon as I have something to review, I'll post it here.

 This change won't require a protocol change and can go into the next
 release.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25700>
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