Logstash 1.5 introduces a powerful feature allowing users to add metadata to events, which remains transient within the Logstash pipeline and is not serialized in outputs, thereby simplifying configuration and enhancing log processing efficiency. This feature facilitates the addition of custom data, filtering, and conditional operations based on metadata without the need for temporary fields. Users can access metadata fields using a specific syntax and leverage them for various tasks, such as optimizing date filters, creating unique identifiers for Elasticsearch documents, and managing intermediate processing results to avoid unnecessary storage. By using metadata, Logstash configurations become less complex, reduce storage requirements, and enhance performance, making metadata a valuable tool in streamlining data processing workflows.