[tor-bugs] #19607 [Metrics/metrics-lib]: avoid repeated keyword strings

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#19607: avoid repeated keyword strings
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 Reporter:  iwakeh               |          Owner:  iwakeh
     Type:  enhancement          |         Status:  needs_review
 Priority:  Medium               |      Milestone:  metrics-lib 1.8.0
Component:  Metrics/metrics-lib  |        Version:
 Severity:  Normal               |     Resolution:
 Keywords:                       |  Actual Points:
Parent ID:                       |         Points:
 Reviewer:                       |        Sponsor:
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Comment (by iwakeh):

 Replying to [comment:17 karsten]:
 > Looks good!
 >
 > I fixed a tiny bug where a space character got lost:
 >
 ...
 > And I added two more commits in an attempt to make it look even better
 and possibly even save memory by storing everything in `EnumSet<Key>`
 rather than `Set<String>`.  Please review
 [https://gitweb.torproject.org/user/karsten/metrics-
 lib.git/log/?h=task-19607-3 my branch task-19607-3] carefully!

 Thanks for fixing the space and adding the missed constants/enums!

 I found another place for use of constants.  In declarations we should use
 interfaces if possible.

 Please find these changes for review
 [https://gitweb.torproject.org/user/iwakeh/metrics-lib.git/?h=task-19607-3
 here (two more commits)].

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19607#comment:18>
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