[tor-bugs] #17830 [metrics-lib]: use StringBuilder correctly

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#17830: use StringBuilder correctly
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 Reporter:  iwakeh       |          Owner:  karsten
     Type:  defect       |         Status:  needs_review
 Priority:  Medium       |      Milestone:
Component:  metrics-lib  |        Version:
 Severity:  Normal       |     Resolution:
 Keywords:               |  Actual Points:
Parent ID:  #17822       |         Points:
  Sponsor:               |
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Comment (by iwakeh):

 this revision looks fine, see #17824 for more.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17830#comment:3>
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