Update iso timestamp to include timezone

[ash-darin]

enhancement

This changes the timestamp for log4j logging into a format, which includes the timezone. This will make it easier for all applications who read the logs to place it correctly without having to guess that this is UTC. Support for this format is documented here:

https://logging.apache.org/log4j/2.x/javadoc/log4j-core/org/apache/logging/log4j/core/util/datetime/FixedDateFormat.FixedFormat.html

This should not break compatibility as the Format is still ISO8601 conforming.

This fixes: #10238

Release notes

Change timestamp in logging to include timezone. Fixes #10238

What does this PR do?

This changes the exposed timestamp in the logstash logs from an ISO timestamp without timezone information to a timestamp with timezone information.

Example:

[2025-06-11T12:14:57,812][INFO ][logstash.agent ] Pipelines running
`
becomes:

[2025-06-11T12:14:57,812+00:00][INFO ][logstash.agent ] Pipelines running
`

The format and setting is explained here:

https://logging.apache.org/log4j/2.x/javadoc/log4j-core/org/apache/logging/log4j/core/util/datetime/FixedDateFormat.FixedFormat.html#ISO8601_OFFSET_DATE_TIME_HHCMM

Note: This is also the timestamp format that kibana uses:

[2025-06-11T15:51:07.413+00:00][INFO ][plugins.actions.server-log]`

Why is it important/What is the impact to the user?

This change allows programs that ingest the logs to correctly autodetect the timezone without having to just assume "UTC" as many programs assume local time without a timezone identifier, see also here:

If no UTC relation information is given with a time representation, the time is assumed to be in local time.

https://en.wikipedia.org/wiki/ISO_8601#Local_time_(unqualified)

Both the old and the new timestamp are ISO8601 format timestrings and all programms supporting ISO8601 parsing should read and interpret the new format automatically.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• [-] I have added tests that prove my fix is effective or that my feature works

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[mergify]

This pull request does not have a backport label. Could you fix it @ash-darin? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[ash-darin]

This pull request does not have a backport label. Could you fix it @ash-darin? 🙏 To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

I'd love to but I am not authorized apparently. Yes, a backport to 8.x would be nice.