Support complex attributes in the Event API

[trask]

Fixes #4199

Allow complex attributes on Events.

Based on discussion in open-telemetry/semantic-conventions#1651 and in today's Specification SIG meeting (recording).

The biggest struggle currently for launching the Event API is the "two property bags" problem, also known as "do I put my event key/value pairs into event attributes or into the event body?"

The Log SIG has been trying to figure out a satisfactory answer to this for a month.

The proposal from @lmolkova is to put everything into event attributes, eliminating one of these "property bags" and answering the question of where to put event key/value pairs.

The event body could still potentially be used for dumping unstructured (or possibly very large) data.

To make this work, we need complex attributes on Events.

This would allow for example "locally scoped" attributes on an event to be defined as a complex attribute in the semantic conventions, e.g. an event named browser.page_navigation_timing could define a complex attribute in the semantic conventions named browser.page_navigation_timing (could be the same as the event name or different), which would contain it's "locally scoped" event attributes underneath it, e.g.

{
  "user.name": ...,
  ...,
  "browser.page_navigation_timing": {
    "name": ...,
    "fetchStart": ...,
    "unloadEventStart": ...,
    "unloadEventEnd": ...,
    ...
  },
  ...
}

[pellared]

A prototype in OTel Go: open-telemetry/opentelemetry-go#6017

The Go SIG strongly prefers: open-telemetry/opentelemetry-go#6018. However, it is also using supporting complex log attributes. Therefore, still an approval from my side as it makes this prototype more aligned.

[austinlparker]

Quick question/clarification - since this strikes me as extending the common attribute definition (https://opentelemetry.io/docs/specs/otel/common/), are we limiting the depth of complex attributes?

[reyang]

We have limits on the number of attributes https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/sdk.md#readablelogrecord. This PR would give people a workaround if they just add everything to a single complex attribute - which is concerning from reliability/performance/security perspective.

[pellared]

This PR would give people a workaround if they just add everything to a single complex attribute - which is concerning from reliability/performance/security perspective.

The attribute limiting issue is already present in the existing Logs (Bridge) API. What is more there are no limits for the Body. More:

• No limits for log record body #4006

[trask]

We discussed in the Log SIG yesterday and decided that these both need to be addressed before stabilizing Events:

• Length limits for complex attribute bodies
• What does "number of attributes" limit mean for logs?

I'll open a tracking issue. EDIT #4335

But we don't think it's a blocker for this PR since it's a pre-existing issue for Log attributes.

[reyang]

But we don't think it's a blocker for this PR since it's a pre-existing issue for Log attributes.

+1