[elasticsearchexporter] Handle new top-level event name field

[felixbarny]

Component(s)

exporter/elasticsearch

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

In open-telemetry/opentelemetry-specification#4260, a new top-level field for EventName has been added to log records. This is set to replace the usage of the event.name attribute, which is now deprecated.

The Elasticsearch exporter should have support for the top-level EventName field.

Describe the solution you'd like

• Serialize LogRecord.EventName as top-level event_name field in OTel mode
• Use attributues["event.name"] as a fallback for event_name, so that consumers (like the UI) can start relying on that field, even if the instrumentations haven't been adapted to use the new field, yet.
• For span events, use event_name in addition to, or instead of attributes.event.name.

Describe alternatives you've considered

We could rely on pre-processing to apply the fallback logic based on the event.name attribute. However, consumers don't have a guarantee over which field is used, as it depends on user configuration.

Additional context

No response

[github-actions]

Pinging code owners:

• exporter/elasticsearch: @JaredTan95 @carsonip @lahsivjar

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[carsonip]

/label -needs-triage