[tor-bugs] #19640 [Metrics/metrics-lib]: review and improve interface hierarchy

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#19640: review and improve interface hierarchy
---------------------------------+-----------------------------------
 Reporter:  iwakeh               |          Owner:  iwakeh
     Type:  enhancement          |         Status:  accepted
 Priority:  Medium               |      Milestone:  metrics-lib 1.9.0
Component:  Metrics/metrics-lib  |        Version:
 Severity:  Normal               |     Resolution:
 Keywords:                       |  Actual Points:
Parent ID:                       |         Points:
 Reviewer:                       |        Sponsor:
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Comment (by iwakeh):

 Metrics-lib parses Tor protocol related descriptors and descriptors from
 other sources.  Shouldn't this distinction be reflected in the interface
 hierarchy?
 Have `Descriptor` on top and introduce an `ExternalDescriptor` interface
 for non-Tor protocol related data sources? Maybe, even have
 `TorDescriptor` too?
 {{{
               Descriptor
             /           \
     TorDescriptor     ExternalDescriptor
 }}}


 This would provide more freedom when for example a switch to an external
 parsing framework (like ANTLR) is made for the Tor related descriptors.
 In addition, non-Tor descriptors could be modeled more easily to the soon
 to come Metrics-data standards and even have their own parsing/generator.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19640#comment:11>
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