Embed fargate data in container samples

[roobre]

Fixes #49

When running in transparent mode, as is the case in Fargate, the agent will not decorate metrics with Fargate metadata such as awsRegion, ecsLaunchType, and ecsClusterName. Breaking queries that use such attributes.

In this PR we fix this by pulling such metadata from the docker container labels that Fargate adds to the container information, by parsing the task arn and the cluster name. This is the same way in which https://github.com/newrelic/nri-ecs obtains this information. RegionFromTask and ResourceNameAndARNBase have been pulled from said repo as well.

TODO:

• Unit testing

Manual testing checklist

• Integration works as expected in Fargate mode, when FARGATE and NRIA_IS_FORWARD_ONLY are set
• Integration works in as expected in EC2 mode
  json-comparison.tar.gz

[CLAassistant]

All committers have signed the CLA.

[gsanchezgavier]

looks good thanks for adding the test

[paologallinaharbur]

Can we use the strings.Split function instead as in the previous method?
Only to avoid splitting using indexes directly

[roobre]

Both functions were taken (i.e. copy-pasted) from nri-ecs, and were left untouched for consistency even when I do not particularly like the implementation of either. The one that uses split, for example, does not perform any kind of bound-checking and relies on the passed string to be an arn.

If we are ok with changing them, I'd vote for rewriting both of them to use strings.Split and proper bound checking. What do you think?

[paologallinaharbur]

Only a small comments, I believe it is fine, but if we can I would use the standard function 👍
Well done!

[roobre]

Following a conversation with @paologallinaharbur, we decided that the best approach for now would be to include the aws sdk even for this tiny bit (arn parsing), and file an issue to switch to said SDK for everything else (hopefully removing all manually vendored files)