Allow STDOUT overrides for AgentBasedEnvironments

[PaulJSchellenberg]

Issue #, if available: None

Description of changes:

Overview

I was looking through the docs and I noticed that if I have the awslogs log driver enabled for ECS and Fargate, I can just output the embedded metric logs to STDOUT. The only way I found to do this was to override the environment to Local and set the required dimensions with environment variables so they don't become Unknown.

Another issue with this method is that I lose all of the container context that the ECSEnvironment provides and I would like to have that.

This change introduces a new environment variable called WRITE_TO_STDOUT where if it's set to true, it will override the AgentSink and return a ConsoleSink in the AgentBasedEnvironment regardless of which AgentBasedEnvironment you use.

I went this direction because it felt like I would need to do some more potentially contentious refactoring around the environments to make an ECSStdoutEnvironment and would need to update the probe in the ECSEnvironment to verify that the cloudwatch agent is available (which I currently don't have the environment setup to test)

Lmk if you think otherwise or if you have another idea to get this done

Testing

• unit testing
• ran the builds

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[gordonpn]

Hi Paul, thank you for the PR.

Could you add a blurb on how this new environment variable will work, under this header on the main README: https://github.com/PaulJSchellenberg/aws-embedded-metrics-java/blob/master/README.md#configuration

Also, how it will affect the existing EnvironmentOverride?

[PaulJSchellenberg]

README updated

[gordonpn]

I've added @markkuhn as a 2nd reviewer.

Once the changes are approved, we'd like to squash merge, please (so that it appears as 1 atomic commit).