Possibility to add optional labels

[andrikaro]

Is your feature request related to a problem? Please describe.

While writing and testing collectors I discovered that when query returns null for a field that is defined under key_labels the query fails and no value is pulled even if it exists.

Describe the solution you'd like

As all of the labels are not strictly required I wonder if it could default to just omitting the label.

Or perhaps there is a way to configure a label as optional.

Describe alternatives you've considered

Currently I found a workaround like this:

key_labels:
  - user
query: |
  SELECT COALESCE(user, '~unknown') FROM table;

Additional context

Of course this issue will be indirectly remedied by this.

[burningalchemist]

Hey @andrikaro, thank you for your suggestion and the workaround posted! 👍

It is tricky to make labels optional as Prometheus devs encourage consistency (which is good anyway). There is a thread in Google Groups specifically about adding/removing labels dynamically for existing metric families. Overall, there's no way to achieve this and if there's one it's discouraged.

What we actually can do is handle null values properly, and return empty strings for labels which aren't populated. I believe this should solve your issue and still keep labels consistent.

I plan to release v0.12.2 this week, and then switch to v0.13 to introduce error metrics. I'll try to test it first, and then probably it becomes a part of the next minor release. 👍

[burningalchemist]

Hey @andrikaro, it's available in v0.12.3, thanks to some regressions. 😅 Please, give it a shot. :)