Add support for fetching container identifier in ECS Fargate

[jsumners-nr]

When an application is running inside of a Docker container, on a Linux guest, our agent is able to retrieve an identifier for that container via cgroups that are populated by Docker. However, when the host is AWS ECS Fargate, those cgroups are not present within the guest. Instead, a metadata API that is available to the guest should be consulted.

We need to:

1. Update the following function to utilize this metadata endpoint if the cgroups are not available:

   node-newrelic/lib/utilization/docker-info.js

   Lines 62 to 88 in 07a841b

   	function fetchDockerVendorInfo(agent, callback) {
   	if (!agent.config.utilization || !agent.config.utilization.detect_docker) {
   	return callback(null, null)
   	}
   	if (vendorInfo) {
   	return callback(null, vendorInfo)
   	}
   	if (!os.platform().match(/linux/i)) {
   	logger.debug('Platform is not a flavor of linux, omitting docker info')
   	return callback(null, null)
   	}
   	// try v2 path first and if null try parsing v1 path
   	common.readProc(CGROUPS_V2_PATH, function getV2CGroup(err, data) {
   	if (data === null) {
   	logger.debug(
   	`${CGROUPS_V2_PATH} not found, trying to parse container id from ${CGROUPS_V1_PATH}`
   	)
   	findCGroupsV1(callback)
   	return
   	}
   	parseCGroupsV2(data, callback)
   	})
   	}

2. We should first attempt to look for the API endpoint through the URI available in the ECS_CONTAINER_METADATA_URI_V4 environment variable. If that environment variable does not exist, try the ECS_CONTAINER_METADATA_URI environment variable. If neither exist, exit with a failure as the function currently does.

• Internal spec: https://source.datanerd.us/agents/agent-specs/blob/12fa39c941badb0de502b07270ab061fd3417cf5/Utilization.md#docker-with-aws-ecs-fargate
• .Net agent implementation: https://github.com/newrelic/newrelic-dotnet-agent/pull/2481/files

[workato-integration]

https://new-relic.atlassian.net/browse/NR-280818