Meta is a field reserved for custom information associated with a log line.
To add metadata to an API call, specify the meta field under the lines object.
Metadata can be viewed inside that line's context.
WARNING: If inconsistent value types are used, that line's metadata, will not be parsed.
For example, if a line is passed with a meta object, such as meta.myfield of type String, any subsequent lines with meta.myfield must have a String as the value type for meta.myfield.
Source: LogDNA official documentation.