Fixes #418 - Metrics Provider

[jantman]

This PR implements the metrics provider subsystem described in #418, as well as the first two metrics providers: a "Dummy" provider that just outputs metrics to STDOUT for testing, and the Datadog provider.

Both current providers have been tested off of this branch and are functional. The process for adding new providers has been documented.

[codecov-io]

Codecov Report

Merging #423 into develop will decrease coverage by 1.67%.
The diff coverage is 67.71%.

@@             Coverage Diff             @@
##           develop     #423      +/-   ##
===========================================
- Coverage      100%   98.32%   -1.68%     
===========================================
  Files           30       33       +3     
  Lines         2319     2444     +125     
  Branches       345      366      +21     
===========================================
+ Hits          2319     2403      +84     
- Misses           0       41      +41

Impacted Files	Coverage Δ
awslimitchecker/metrics/dummy.py	100% <100%> (ø)
awslimitchecker/checker.py	100% <100%> (ø)	⬆️
awslimitchecker/runner.py	100% <100%> (ø)	⬆️
awslimitchecker/metrics/base.py	100% <100%> (ø)
awslimitchecker/metrics/datadog.py	24.07% <24.07%> (ø)

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