Add event name and severity considerations to log-based exception events

[lmolkova]

Fixes #3100

Extracted from open-telemetry/opentelemetry-specification#4333

Related to #3256

Summarizes guidance behind: #3414, #3426, #3435, #3436

The current guidance on recording exceptions on logs only tells how to attach exception-related details to a log record.
OTel instrumentations, when switching to log-based events from span events, would need to emit the whole record and need guidance on:

• whether to emit log or event. This PR recommends events.
• how to name events - use precise names like http.client.request.exception. OTel instrumentations that emit them should document them in central or instrumentation-specific docs.
• how to set severity. This PR suggests to use Fatal or Error only when exception is not handled by application code. I.e. instrumentations that create server/consumer spans would be ones to ever use error severity. Client and producer should emit warn.

[github-actions]

This PR has been labeled as stale due to lack of activity. It will be automatically closed if there is no further activity over the next 7 days.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lmolkova / name: Liudmila Molkova (87d12ec, 8d40073, a7dc1c7, d20a272, ea22175)
• ✅ login: trask / name: Trask Stalnaker (a7dc1c7)

[trask]

It should be obvious, but maybe worth being explicit about capturing span context if one exists.

[pellared]

🏅