Add post-timer labels (#1)

[notkevin888]

One common use case I've run into while using Prometheus at Tinder is having a label for success/failure on a single metric, like the following:

var promClient = require('prom-client');
var metric = promClient.metric('metric_name', 'metric_help', ['success_response']);

var timer = metric.startTimer();
someFunction((err, result) => {
  if (err) {
    timer({ success_response: 'ERROR' });
    console.log('uh-oh');
    // do error logic
  } else {
    timer({ success_response: 'SUCCESS' });
    console.log('yay!');
    // do success logic
  }
})

With the original code, it seemed like there wasn't a very clean way to handle this; the only way we had solved the problem before was by creating two timers (eg, a successTimer and an errorTimer) and only ever calling one of the timers later, but that seems both prone to error and not very memory-safe.

In addition, we might also want to do something with more than just 2 options, such as tracking average response times for every HTTP code we have in an application. In this case, the multiple-timer approach is also not very scalable, since we'd have to add an additional timer for each new code we wanted to send.

This was the approach I came up with; let me know if there's a better way of handling these kinds of use cases.

[siimon]

Great, thanks! Would you mind to add some tests for this and update the readme?

I won't be able to publish to npm until the middle of next week since I'm away from a computer - hope that's ok!

[notkevin888]

Yeah, for sure :) Adding those in now!

[notkevin888]

Hey, hope you had a great weekend! I know it's still the beginning of the week; just wondering what the timeline looks like for this pr making it to npm :)

[siimon]

Published with version 3.5.0! Thanks for your help!

[notkevin888]

Sweet, thank you!