Loggregator Agent 3.11

Changelog

• Expose Metrics via Promethus endpoints rather than Expvar in Syslog Agent, Forwarder Agent, and Loggregator Agent.
• Update Golang Version to 1.12.4
• System Metrics Agent provides per-core metrics for CPU
• Add indicators for Syslog Agent
• Property change: loggr-forwarder-agent and loggr-syslog-agent debug_port changed to pprof_port
• Fix envelope tagging from UDP Forwarder