For compatibility with the V2 agent control protocol, beats run under agent should start without loading a configuration file from disk. Preferably a beat run under agent should start, establish a V2 control protocol client, and wait for an initial configuration from the agent.

This issue is cloned from @michalpristas's original comment in the V1 protocol removal issue: elastic/elastic-agent-client#32 (comment)

Beats behaviour change

Current

At the moment we handle process as this:

• elastic agent starts
• elastic agent is notified about or detects config change
• elastic agent creates a specific change for specific beats (let's assume filebeat from now on)
• elastic agent starts filebeat process with -management.enabled flag
• filebeat process starts
• filebeat loads a config from filebeat.yml

	cfg, err := cfgfile.Load("", settings.ConfigOverrides)
	if err != nil {
		return fmt.Errorf("error loading config file: %w", err)
	}

• filebeat initializes manager to get comms with agent going

	// initialize config manager
	b.Manager, err = management.Factory(b.Config.Management)(b.Config.Management, reload.Register, b.Beat.Info.ID)
	if err != nil {
		return err
	}

• filebeat fetches config from fleet

Proposed

• elastic agent starts
• elastic agent is notified about or detects config change
• elastic agent creates a specific change for specific beats (let's assume filebeat from now on)
• elastic agent starts filebeat process with -management.enabled flag
• filebeat process starts
• in case management.enabled flag is not set filebeat loads a config from filebeat.yml otherwise use sane default configuration

	cfg, err := cfgfile.Load("", settings.ConfigOverrides)
	if err != nil {
		return fmt.Errorf("error loading config file: %w", err)
	}

• filebeat initializes manager to get comms with agent going

	// initialize config manager
	b.Manager, err = management.Factory(b.Config.Management)(b.Config.Management, reload.Register, b.Beat.Info.ID)
	if err != nil {
		return err
	}

• filebeat fetches config from fleet2