[metrics-bugs] #21578 [Metrics/Metrics website]: Resolve deprecation warnings after upgrading to metrics-lib 1.6.0

[Tor Bug Tracker & Wiki]

#21578: Resolve deprecation warnings after upgrading to metrics-lib 1.6.0
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     Reporter:  karsten                  |      Owner:  metrics-team
         Type:  defect                   |     Status:  new
     Priority:  Medium                   |  Milestone:
    Component:  Metrics/Metrics website  |    Version:
     Severity:  Normal                   |   Keywords:
Actual Points:                           |  Parent ID:
       Points:                           |   Reviewer:
      Sponsor:                           |
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 See summary, will attach branch in a minute.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21578>
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