Add health check function to a circuit breaker

[lance]

Add circuit.healthCheck(func[, interval])

Exposes a way for users to provide their own health check function. The provided function
should return a promise. If the promise is rejected, the circuit will open and the health-check-failed event will be emitted.

Usage

const circuit = opossum(myFunction);
circuit.healthCheck(_ => {
    // check the status of the remote service
    if (remoteServiceDown) {
       return Promise.reject('Cannot connect to remote service');
    }
    return Promise.resolve();
  }, 10000); // check interval - default is 5000

circuit.on('health-check-failed', (err) => {
  // send a text message to the poor sysadmin
  sendErrorText(err);
});

[coveralls]

Coverage increased (+0.02%) to 99.396% when pulling 0088ea8 on 52-health-check into 5b6bc96 on master.

[helio-frota]

Good !
In fact I did not look in impl detail, but the usage seems very easy for the end user. 👍

[lholmquist]

I wonder if it also makes sense to have a Health check pass event.

I have no argument for or against :)

[lance]

I thought about it, but it seems like a lot of unnecessary noise. Since there are only two possible states, pass or fail, it seems like the absence of fail implies pass.

[lholmquist]

Yea, probably no reason to have it. I'd say merge away 🎉

[lholmquist]

related to my comment about the health-check-passed event, should we do something on non error

[lance]

Kind of have the same reasoning about this as I do about health-check-passed.