[metrics-bugs] #20224 [Metrics/CollecTor]: Fix `BridgeDescriptorMappingsLimit` config option

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#20224: Fix `BridgeDescriptorMappingsLimit` config option
-------------------------------+---------------------------------
 Reporter:  karsten            |          Owner:  karsten
     Type:  defect             |         Status:  accepted
 Priority:  Low                |      Milestone:  CollecTor 1.7.0
Component:  Metrics/CollecTor  |        Version:
 Severity:  Normal             |     Resolution:
 Keywords:  metrics-2018       |  Actual Points:
Parent ID:                     |         Points:
 Reviewer:                     |        Sponsor:
-------------------------------+---------------------------------
Changes (by karsten):

 * owner:  metrics-team => karsten
 * status:  assigned => accepted


Comment:

 Taking over from iwakeh here. irl, please review
 [https://gitweb.torproject.org/karsten/metrics-db.git/log/?h=task-20224-2
 my task-20224-2 branch], which is a rebase of iwakeh's branch linked above
 plus a squash commit with two fixes.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20224#comment:14>
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