[tor-bugs] #19021 [Metrics/CollecTor]: improve configuration process

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#19021: improve configuration process
-----------------------------------+----------------------------
     Reporter:  iwakeh             |      Owner:  iwakeh
         Type:  enhancement        |     Status:  new
     Priority:  Medium             |  Milestone:
    Component:  Metrics/CollecTor  |    Version:
     Severity:  Normal             |   Keywords:  ctip operation
Actual Points:                     |  Parent ID:
       Points:                     |   Reviewer:
      Sponsor:                     |
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 Sketch of the new configuration process:
 * use `Properties` to store and read the configuration parameters.
 * read external configuration properties file (either local path or via
 command line argument).
 * provide a default configuration as resource within the executable jar.
 * if there is no external configuration file present, write out the
 default values.
 * re-read properties when file changes; only between module runs (this is
 after the implementation of the scheduler, #19018)
 * use enum as keys

 Questions to decide:
 1. property format: XML or plain properties (the latter might be more
 readable)
 1. default value settings

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19021>
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