Populate `event_sequence` field for each Collector message

The `event_sequence` field can be used as secondary sort field for messages that have the same millisecond timestamp.

For files we can use the `log.file.record_number` or `log.file.record_offset`, for other sources we need to find a field. We can probably use one of the nanosecond timestamps in OTel (`Timestamp` or `ObservedTimestamp`) as a fallback.

The content team will add the `event_sequence` field with type `long` as official GIM field for 7.2.

Start by tracing how Collector messages are constructed and how source-specific metadata is attached. Compare file records using log.file.record_number or log.file.record_offset with other sources and the proposed OTel Timestamp or ObservedTimestamp fallback. Done means every Collector message populates event_sequence as a long, consistent with the planned GIM field.

Written by the indexing model from the issue text.