[20181026012755] Metrics API, Rack URL Injection, and Bug Fixes

[ddollar]

This release adds cluster and app metrics to the Rack API and allows you to inject a RACK_URL environment variable into your apps by setting the app parameter RackUrl=Yes

• #2933 metrics api [@ddollar]
• #2993 use t2 instances for rack defaults [@ddollar]
• #2994 prevent env defaults from leaking between services [@ddollar]
• #2996 optionally add RACK_URL to application environment [@ddollar]