Only exposes one tag in __meta_tailscale_device_tag

[antifuchs]

I have some machines with multiple tags that I would love to make rules for (service-discover only a specific set of machines that I know expose a prometheus metrics service), but unfortunately it looks like tailscalesd flattens the set of tags into a single value - for now, I can make do, but it would be great if it collected all the tags into a structure that allows matching on all the tags.

[cfunkhouser]

Considering a machine with tags tag:foo and tag:bar, TailscaleSD currently generates two target entries, like the following (with other labels trimmed for brevity):

[
  {
    "targets": [
      "100.93.73.3"
    ],
    "labels": {
      "__meta_tailscale_device_tag": "tag:foo"
    }
  },
  {
    "targets": [
      "100.93.73.3"
    ],
    "labels": {
      "__meta_tailscale_device_tag": "tag:bar"
    }
  }
]

@antifuchs Would it be helpful to have a single target with labels including the tag names?

[
  {
    "targets": [
      "100.93.73.3"
    ],
    "labels": {
      "__meta_tailscale_device_tag_foo": true,
      "__meta_tailscale_device_tag_bar": true
    }
  }
]

[SuperSandro2000]

I would expect the example to be returned like:

[
  {
    "targets": [
      "100.93.73.3"
    ],
    "labels": {
      "__meta_tailscale_device_tag": "tag:bar,tag:foo"
    }
  }
]

I did not think about that to long, but it felt right in the first thought.

[antifuchs]

I like the "one label per tag" approach @cfunkhouser proposes. The comma-separated thing makes it more difficult to match things with promql (have to use regexes, be aware of whether you're matching the first tag, middle tag or last tag). Less parsing would be better here (:

I didn't even notice that multiple targets were created for multiple tags - that does sound like a pretty bad time if a consumer doesn't expect that.

[cfunkhouser]

@SuperSandro2000 I decided to go with the label-per-tag approach for the reasons @antifuchs suggests. I replaced true with "1" as the static value, however. For example, the new behavior will produce:

[
  {
    "targets": [
      "100.93.73.3"
    ],
    "labels": {
      "__meta_tailscale_device_tag_foo": "1",
      "__meta_tailscale_device_tag_bar": "1"
    }
  }
]

If the static value being a string becomes annoying, it can be fixed. However, given the way the JSON is generated now, it would have been a more annoying change than I wanted to tackle.

If either of you are willing to try things out before I cut a release, I would appreciate feedback!