CloudEvents: relax requirement levels

[lmolkova]

While CloudEvents spec requires most of the required attributes to be present on the event, not all of them may be required for telemetry purposes.

event_source + event_id would uniquely identify the event, so let's keep them required.

For others requirement level could potentially be relaxed:
- event_subject is optional per CloudEvents spec

• event_spec_version is probably mostly static and rarely changed. Making it recommended or optional seem to be more reasonable
• event_type: can it become recommended?

Recommended attributes would still be populated by default unless there are strong reasons not to. Instrumentation may provide a way to disable them explicitly.

[lmolkova]

@joaopgrassi what do you think? I can do the change if you're ok with it.

[joaopgrassi]

Hi @lmolkova !

event_source + event_id would uniquely identify the event, so let's keep them required.

👍

About the others:

• event_subject is optional as you said, so no changes here
• event_type as recommended: Sounds reasonable to me!
• event_spec_version: I'm divided a bit on this one. It's probably static as you said, but if there's cross-systems communications and they use different versions, by using optional we might miss the info that could help in observing things? So maybe recommended is better? It's always present in the event so, should not have any performance penalty in adding/looking it up.

[lmolkova]

event_subject is optional as you said, so no changes here

ah, right! sorry for the confusion

event_spec_version

recommended sounds reasonable and I think we can suggest skipping if default (1.0)

[joaopgrassi]

suggest skipping if default (1.0`)

Not sure, do we really need to skip it? What is your concern in always having it? My rationale is it's a required attribute in the CloudEvents spec, so making it optinal/not always present here might be odd since it goes against what it's in their spec. I know we are not the same thing but.. maybe keeping it consistent is better? And if people really think it's unecessary, using recommended they can turn it off, right?

[lmolkova]

@joaopgrassi the only worry is redundancy: while only 1.0 is available, there is not much gain in populating it and lack of it can just mean default value. But I agree that recommended is good enough and no need to go further.

[joaopgrassi]

Sounds good then! Would you submit the PR? Or should I do it? Either is fine with me. Feel free to ping me for the approval if you open :)

[lmolkova]

@joaopgrassi Thanks! I'll send the PR, I want to merge #2594 first so I can use new requirement levels here properly.