fix: Support OTLP event_name field in logs exporter

[putcho01]

Summary

fix: #1078

Problem

The event_name field was added to the OTLP LogRecord proto in open-telemetry/opentelemetry-proto#600, but the logs exporter was not handling this field. Previously, event names were set as event.name attributes and were captured in labels. With the new dedicated proto field, this information was being dropped.

Solution

Map the OTLP event_name field to the Cloud Logging LogEntry label "event.name". This approach:

• Maintains backward compatibility with the previous convention of using event.name as an attribute
• Works for all log payload types (text, JSON, etc.)
• Allows easy querying via labels."event.name" in Cloud Logging

[dashpole]

/gcbrun

@DylanRussell how did you handle Event name in the python SDK log exporter? We should probably try to be consistent here.