feat(rust): add some metrics backends

[dbanda]

Switches the metrics client to be a trait and introduces a testing and datadog backend of the metrics client. Datadog is still WIP but this impl uses the dogstatsd library.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (a29e615) 90.34% compared to head (c747139) 90.34%.

:exclamation: Current head c747139 differs from pull request most recent head 14d6aa8. Consider uploading reports for the commit 14d6aa8 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4063   +/-   ##
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  Coverage   90.34%   90.34%           
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  Files         803      803           
  Lines       39514    39514           
  Branches      245      245           
=======================================
  Hits        35698    35698           
  Misses       3784     3784           
  Partials       32       32           

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[MeredithAnya]

I think we already have metrics now