fix: emit warn when labels and values are out of sync

[leftieFriele]

This PR improves the guard by emitting warn messages when the labels or values are undefined, which should make for less errors in application when requests are terminated or similar things happens.

Previously this would result in the error mentioned in #53

Closes #53

[wkillerud]

The name is one thing, but can the value not be null or undefined? 🤔

[leftieFriele]

that is what we want to warn against, as you will get the error about arguments and labels not matching if value if null of undefined. right?

[wkillerud]

For labels yes, but I think a label's value can be falsy and not fail. I can write a test.

[wkillerud]

I added this test to histogramTest.js in prom-client and it passes:

it('should not crash on falsy values', () => {
  let i = new Histogram({
    name: 'histo',
    help: 'help',
    labelNames: ['code'],
  });
  i = i.labels(undefined);
  expect(true).toEqual(true);
});

This one fails because we get two values for the one configured label name:

it('should not crash on falsy values', () => {
  let i = new Histogram({
    name: 'histo',
    help: 'help',
    labelNames: ['code'],
  });
  i = i.labels(undefined, undefined);
  expect(true).toEqual(true);
});

[leftieFriele]

👍 I'll add a test for this here as well.

[wkillerud]

A bit off topic, but these inline messages from GitHub are not helping readability 🙈

[trygve-lie]

Should we just scan all object properties for null / undefined values instead of targeting specific properties?

[leftieFriele]

sounds like a good idea 👍

[leftieFriele]

I was looking at the types for MetricLabel and there null is valid for values, so I guess we should only look for undefined then.

[leftieFriele]

I looked at this some more, metric.labels is an array of MetricLabels and that means I shouldn't have to look for anything but the name and value should as @wkillerud said be allowed to be null / undefined

[wkillerud]

What I meant with these unit tests in prom-client was that undefined and null should be allowed as a value.

[wkillerud]

An undefined value should be allowed in my opinion, and not emit a warning.