Work with elastic-agents

[xli]

Current Java agent accept a java properties file to setup and run in elastic-agents, golang agent uses environment variables. Because elastic-agents are in development, so we will wait until we have a stable elastic-agents implementation, and decide what to do. Probably it's just as easy as read java properties file and setup environment variables for this agent.

[tomzo]

I think variables mentioned in readme can stay as they are. It would suffice to add reading of ${GOCD_AGENT_CONFIG_DIR}/autoregister.properties file before registration begins.

After considering elastic agents properties file contains

agent.auto.register.key
agent.auto.register.hostname
agent.auto.register.environments 
agent.auto.register.resources 
agent.auto.register.elasticAgent.pluginId
agent.auto.register.elasticAgent.agentId

I want to add support for setting up golang agent with go cookbook, but I see there is no way do autoregistration at this point.

[tomzo]

Sorry, I just found out that all these values are already supporeted by environment variables. So this issue is just about semantics of the input file.

[djrice]

The docker elastic agents plugin definitely does need to send different env vars to the agent containers. Right now it sends the contents of autoregister.properties in a single, multi-line variable rather than each as a separate env var. But that's an issue for the docker elastic agent plugin repository.