[metrics-bugs] #22141 [Metrics/metrics-lib]: Deprecate `DescriptorFile` and add relevant information to `Descriptor`

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#22141: Deprecate `DescriptorFile` and add relevant information to `Descriptor`
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     Reporter:  karsten              |      Owner:  metrics-team
         Type:  enhancement          |     Status:  new
     Priority:  Medium               |  Milestone:
    Component:  Metrics/metrics-lib  |    Version:
     Severity:  Normal               |   Keywords:
Actual Points:                       |  Parent ID:
       Points:                       |   Reviewer:
      Sponsor:                       |
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 I'd want us to implement #20395 where we'd be able to handle much larger
 descriptor files without copying all contents to memory before even
 looking at them.  But I realized that `DescriptorFile#getDescriptors()`
 makes this rather pointless.  If we need to keep a list with all parsed
 descriptors in memory, each containing raw contents of some sort (see
 #22140), then what's the point of avoiding to read complete files to
 memory?

 One way to fix this is to deprecate `DescriptorFile` and add all relevant
 information to `Descriptor`.  And then `DescriptorReader` would return an
 `Iterator<Descriptor>`, internally backed by
 `BlockingIteratorImpl<Descriptor>`, rather than an
 `Iterator<DescriptorFile>`.  Sounds easy!

 Here's a catch though: `DescriptorFile#getException()` returns "the first
 exception that was thrown when reading this file or parsing its content".
 If we'd move this method to `Descriptor`, would we set an I/O exception to
 the first descriptor in that file, to all of them, or maybe to none of
 them?  Turns out we don't have to worry too much about this.  The only
 code that actually uses `DescriptorFile#getException()` is Onionoo's
 `DescriptorQueue`, and all it does is log the exception.  We could just do
 the same.

 What else could go wrong?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22141>
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