introduce debug.id helper method

[indutny]

When debugging the applications that work with pools of streams
or sockets, the debug stream often comes out interleaved making it
impossible to distinguish between streams. It is a common knowledge
that unique ids could be assigned to each stream and be used in
logging to solve this. However, this solution is rather ad-hoc and
usually applied only during manual debugging.

This commit introduces standard function debug.id() that
increments the internal counter only when debugging is enabled, and
otherwise returns undefined.

---

This have bugged me any times, but realization that it could be done in one place and for all came just now.

As a more questionable alternative, I think it could work differently too:

const debug = debug('app');

class Instance {
  constructor() {
    this.debug = debug.id('id=%d msg: %s');
  }

  method() {
    this.debug('message params=%j', {});
    // "app id=123 msg: message params={}"
  }

  workWithAnotherInstance(other) {
    this.debug('work with=%d', other.debug.id);
    // "app id=123 msg: work with=456"
  }
}

What do you think?

[indutny]

Going to open a second PR for alternative.

[coveralls]

Coverage increased (+2.6%) to 76.331% when pulling 1141cf5 on indutny:feature/debug-id into 22f9932 on visionmedia:master.

[coveralls]

Coverage increased (+2.6%) to 76.331% when pulling 1141cf5 on indutny:feature/debug-id into 22f9932 on visionmedia:master.

[Qix-]

Hello @indutny :)

What does the ID function solve? I mean, what would consume it?

[Qix-]

#544 (comment)