A note on the Metadata propertyedit
The C# EcsDocument type includes a property called Metadata with the signature:
/// <summary>
/// Container for additional metadata against this event.
/// </summary>
[JsonPropertyName("metadata"), DataMember(Name = "metadata")]
public IDictionary<string, object> Metadata { get; set; }
This property is not part of the ECS specification, but is included as a means to index supplementary information.