[Logs] Recommend setting Severity Number

[pellared]

Area(s)

area:log

What's missing?

Logs without severity are bad. It's expensive to manage logs when you don't know if they can be dropped/sampled or not.

Original Slack thread: https://cloud-native.slack.com/archives/C062HUREGUV/p1762896560429319

Describe the solution you'd like

It would be nice to encourage users to always set the Severity Number. This could be done by adding "general logs semantics" similarly to general event semantics.

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[trask]

Discussed in Log SIG:

we think this probably belongs in spec, something like

Logs SHOULD have a severity number

Related, from https://github.com/open-telemetry/semantic-conventions/blob/main/docs/general/events.md#general-event-semantics:

Events SHOULD specify a severity number.

we think Events SHOULD have a severity number (this part in spec)

and change above in semconv to

Events SHOULD specify a default severity number.

and one more thought, future convenience Log API could require severity number

[pellared]

we think this probably belongs in spec, something like

Logs SHOULD have a severity number

We cannot place this requirement in the OpenTelemetry specification (API/SDK/Data Model) because the severity number is defined as optional. Stating that it SHOULD be specified would contradict the existing design, where the field is intentionally optional.

On the other hand, semantic conventions target telemetry producers, and that is where we can recommend that a severity number SHOULD be set. For this reason, I believe such guidance belongs in the Semantic Conventions rather than in the core specification.

I think this issue is very low priority right now as we seem to prefer that users emit event records over log records. On the other hand, maybe we should have some recommendation for log bridge/appender authors that having unset severity level is a very bad practice.