feat: refactor Prometheus and Hystrix metrics (#350)

BREAKING CHANGE

Instead of continuing to add different metrics modules to the
circuit breaker API, we can extract the `PrometheusMetrics` and
`HystrixStats` classes as their own modules. All they need is a
handle to the circuit itself so they can listen for events and/or
monitor the circuit stats. This allows us to continue maintaining
things like Hystrics that are deprecated upstream, without polluting
our circuit breaker implementation.

Hystrix and prometheus metrics are now handled by external modules
that are in nodeshift/opossum-hystrix and nodeshift/opossum-prometheus.

README.md

@@ -241,55 +241,15 @@ If you'd like to add them, run `npm install @types/opossum` in your project.
 ### Metrics
 
 #### Prometheus
-Provide `{ usePrometheus: true }` in the options when creating a circuit to produce
-metrics that are consumable by Prometheus. These metrics include information about
-the circuit itself, for example how many times it has opened, as well as general Node.js
-statistics, for example event loop lag. To get consolidated metrics for all circuits in your
-application, use the `metrics()` function on the factory. 
+The [`opossum-prometheus`](https://github.com/nodeshift/opossum-prometheus) module
+can be used to produce metrics that are consumable by Prometheus.
+These metrics include information about the circuit itself, for example how many
+times it has opened, as well as general Node.js statistics, for example event loop lag.
 
-```js
-const opossum = require('opossum');
-
-// create a circuit
-const circuit = opossum(functionThatMightFail, { usePrometheus: true });
-
-// In an express app, expose the metrics to the Prometheus server
-app.use('/metrics', (req, res) => {
-  res.type('text/plain');
-  res.send(opossum.metrics());
-});
-```
-
-The `prometheusRegistry` option allows to provide a existing 
-[prom-client](https://github.com/siimon/prom-client) registry. 
-The metrics about the circuit will be added to the provided registry instead 
-of the global registry.
-The [default metrics](https://github.com/siimon/prom-client#default-metrics) 
-will not be added to the provided registry.
-
-```js
-const opossum = require('opossum');
-const { Registry } = require('prom-client');
-
-// Create a registry
-const prometheusRegistry = new Registry();
-
-// create a circuit
-const circuit = opossum(functionThatMightFail, {
-  usePrometheus: true,
-  prometheusRegistry
-});
-```
 
 #### Hystrix
-
-**NOTE: Hystrix metrics are deprecated**
-
-A Hystrix Stream is available for use with a Hystrix Dashboard using the `circuitBreaker.hystrixStats.getHystrixStream` method.
-
-This method returns a [Node.js Stream](https://nodejs.org/api/stream.html), which makes it easy to create an SSE stream that will be compliant with a Hystrix Dashboard.
-
-Additional Reading: [Hystrix Metrics Event Stream](https://github.com/Netflix/Hystrix/tree/master/hystrix-contrib/hystrix-metrics-event-stream), [Turbine](https://github.com/Netflix/Turbine/wiki), [Hystrix Dashboard](https://github.com/Netflix/Hystrix/wiki/Dashboard)
+The [`opossum-hystrix`](https://github.com/nodeshift/opossum-hystrix) module can
+be used to produce metrics that are consumable by the Hystrix Dashboard.
 
 ## Troubleshooting
 
@@ -303,7 +263,7 @@ You may run into issues related to too many listeners on an `EventEmitter` like
 (node:25619) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 finish listeners added. Use emitter.setMaxListeners() to increase limit
 ```
 
-This typically occurs when you have created more than ten `CircuitBreaker` instances. This is due to the fact that every circuit created is adding a listener to the stream accessed via `circuit.stats.getHystrixStream()`, and the default `EventEmitter` listener limit is 10. In some cases, seeing this error might indicate a bug in client code, where many `CircuitBreaker`s are inadvertently being created. But there are legitimate scenarios where this may not be the case. For example, it could just be that you need more than 10 `CircuitBreaker`s in your app. That's ok.
+In some cases, seeing this error might indicate a bug in client code, where many `CircuitBreaker`s are inadvertently being created. But there are legitimate scenarios where this may not be the case. For example, it could just be that you need more than 10 `CircuitBreaker`s in your app. That's ok.
 
 To get around the error, you can set the number of listeners on the stream.