added a metrics package and moved metrics files to package

[maurafortino]

What's Included:

• created a metrics package
  • moved metrics.go, metrics_test.go and anclaHelper.go files to the metrics package
  • moved CaduceusMetricsRegistry interface to metrics.go file (import issues if left in main)
  • moved CreateOutbounderMetrics to outboundSender.go (import issues if left in metrics package)
• moved error reasons constants to http.go (all but one are only related to http.go - only error related to metrics is network error)
• all other updates are just updating files with the metrics package and differentiating between caduceus metrics package and gokit metrics package.

[denopink]

I made a quick change. otherwise, lgtm!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (denopink/chore/restructure-code-base@fbffc52). Click here to learn what that means.
The diff coverage is n/a.

@@                           Coverage Diff                           @@
##             denopink/chore/restructure-code-base     #428   +/-   ##
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  Coverage                                        ?   66.12%           
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  Hits                                            ?      855           
  Misses                                          ?      408           
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Flag	Coverage Δ
unittests	66.12% <0.00%> (?)

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