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

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#19640: review and improve interface hierarchy
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 Reporter:  iwakeh               |          Owner:  metrics-team
     Type:  enhancement          |         Status:  assigned
 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):

 Also from #21932 comments 14/15, listed for discussion:
 * avoid constant duplication, think about adding or not adding certain
 constants to the api (refers to DescriptorImpl.NL and ExitList.EOL)
 * add overloaded methods for `newScanner`, b/c the usual delimiter is "\n"
 * DescriptorImpl.setDigestXXX allow empty or null argument.  This should
 have a check even though currently the calling methods make sure the
 argument is not null or empty, but when working on other tasks in the
 future that might not be apparent anymore and would get lost w/o a check
 and accompanying test.
 * Think about improving TorperfResultImpl and ExitListImpl (also in regard
 to delimiter use, see question 4, comment 14, #21932).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19640#comment:9>
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