GELF message seems to be missing mandatory field "version"

[mikknormak]

GELF message format:
https://docs.graylog.org/en/latest/pages/gelf.html
defines:

version string (UTF-8)
GELF spec version – “1.1”; MUST be set by client library.

https://github.com/mp911de/logstash-gelf/blob/main/src/main/java/biz/paluch/logging/gelf/intern/GelfMessage.java
des use

GELF_VERSION_1_1.equals(version)

to define version specific "timestamp" and "level" formats, but does not seem to include a "version" field into final JSON message.
(and since all custom field are prefixed with "_" there is also no (easy) way to manually provide "version" field)

perhaps something like this:

public static final String FIELD_VERSION = "version";
...
if (GELF_VERSION_1_1.equals(version)) {
hasFields = writeIfNotEmpty(out, hasFields, FIELD_VERSION, GELF_VERSION_1_1);
}

or perhaps more future-proof:

public static final String FIELD_VERSION = "version";
...
if (!isEmpty(version) && !GELF_VERSION_1_0.equals(version)) {
hasFields = writeIfNotEmpty(out, hasFields, FIELD_VERSION, version);
}

in case GELF_VERSION_1_0 must not include the "version" field

[mp911de]

Good catch. I think the second variant makes sense. Do you want to submit a pull request?

[mikknormak]

Sure:
#260