Add a DISABLE_METRIC_EXTRACTION config option

[Dunedan]

This adds a new configuration option DISABLE_METRIC_EXTRACTION which
removes the _aws metadata from the EMF log records, thus disabling the
extraction of metrics by CloudWatch Metrics. The remaining part of the
log records is still getting logged.

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Fixes #34

[jaredcnance]

Thanks for the PR! Build is currently failing due to:

aws_embedded_metrics/serializers/log_serializer.py:23:1: E303 too many blank lines (3)
118 | ERROR: InvocationError for command /codebuild/output/src419401436/src/github.com/awslabs/aws-embedded-metrics-python/.tox/flake8/bin/flake8 aws_embedded_metrics tests (exited with code 1)
119
...
aws_embedded_metrics/serializers/log_serializer.py:42: error: Incompatible types in assignment (expression has type "Dict[str, List[Dict[str, object]]]", target has type "str")
--
126 | Found 1 error in 1 file (checked 24 source files)
127 | ERROR: InvocationError for command /codebuild/output/src419401436/src/github.com/awslabs/aws-embedded-metrics-python/.tox/mypy/bin/mypy aws_embedded_metrics (exited with code 1)

[Dunedan]

I fixed those issues now and locally tests run fine, however on CodeBuild they still seem to fail. No idea why, because I can't access the CodeBuild output.

[jaredcnance]

Tests failed due to a race condition in one of the agent tests where the socket wasn't available for re-use yet.