Add Runtime config API

[nmittler]

Title: Add Runtime config API

Description:
As part of some ongoing work in Istio to support Spring Boot applications using Hystrix circuit breakers, there is a desire to allow the application to override the circuit breaking configuration of it's Envoy instance.

I think to be safe, we'd want to be able to change any of Istio's circuit breaker params:

• cluster.MaxRequestsPerConnection
• cluster.CircuitBreaker.Default.MaxConnections
• cluster.CircuitBreaker.Default.MaxRequests
• cluster.CircuitBreaker.Default.MaxPendingRequests
• cluster.OutlierDetection.MaxEjectionPercent
• cluster.OutlierDetection.BaseEjectionTimeMS
• cluster.OutlierDetection.ConsecutiveErrors
• cluster.OutlierDetection.IntervalMS
• cluster.OutlierDetection.MaxEjectionPercent

Relevant Links:
Related Discussion
Istio's circuit breaker params

[mattklein123]

I suggest handling this via #514. All of the above settings are currently configurable (or will be with a few minor alterations) via the runtime system built into Envoy. If we had an easy to use admin API which allowed you to query/set runtime keys would that satisfy what you need? Then you could hit the local API via an interceptor.

[nmittler]

@mattklein123 whatever you think ... I only raised this issue because you had suggested it.

[mattklein123]

@nmittler I'm asking if the above solution will satisfy you. :)

[nmittler]

@mattklein123 yeah AFAIK that should work

[mattklein123]

OK I'm going to resolve this in favor of #514 as that will solve your issue and is already open.

[nmittler]

@mattklein123 SGTM, thanks!