Distinguish types of metrics into Event/Config/Utilization

[mattrjacobs]

I think that there are 3 categories of metrics that Hystrix exposes, and they are mixed together at the moment (up to 1.4.x).

The 3 types are:

• Events (Success, Short-Circuited, Fallback Success, Collapsed, etc). These make sense in the context of a single request, or aggregated. They are discrete values that occur at a point in time.
• Config (Command timeout values, Mappings of Commands -> ThreadPools, etc). These are JVM-scoped, and can be sampled at any point. These values exist in a given request, but are more accurately viewed as continuous values that change fairly slowly/infrequently
• Utilization (Command concurrency, Thread Pool concurrency, etc). These are also JVM-scoped like config, but change much more rapidly (as commands enter/leave the system). They are also continuous, and may be sampled. They don't fit into a concept of a request naturally.

At the moment, when a metrics publisher collects values, it combines these 3 concepts. It looks at aggregated event metrics and samples config and utilization values. I believe that distinguishing these offers more flexibility.

In an ideal world, I would publish:

• Event metrics on each request completion (including event type and latency)
• Config metrics sampled very infrequently
• Utilization metrics sampled very frequently

This assumes the existence of a system which can consume a high volume of data and do aggregation off-box. So it certainly is not for every Hystrix user. Therefore, I intend to leave the existing metrics pathways as-is, so I don't break any current users of Hystrix.

This issue will track the addition of:

• Request-based event stream
• Config sample stream
• Utilization sample stream

[mattrjacobs]

These 3 streams are added in the following form:

• Utilization sample stream, served by SSE and returning text/event-stream JSON (1x per poll interval)
• Configuration sample stream, served by SSE and returning text/event-stream JSON (1x per poll interval)
• Requests stream, served by SSE. Returns [0..n] requests as text/event-stream JSON (1x per poll interval)