[zabbix] Add Linux: prefix support for Zabbix 6.2+ template item names

[shin25818]

Summary

Zabbix 6.2 (released July 2022) changed the naming convention of template
item names by adding prefixes such as "Linux: " to item names.

For example:

• Before 6.2: Load average (1m avg)
• After 6.2: Linux: Load average (1m avg)

The current METRICS list only contains the old naming convention, causing
Metric Samples to remain at 0 for all users running Zabbix 6.2 or later.

Changes

Added Linux: prefixed versions of existing metric mappings to support
Zabbix 6.2+ while maintaining backward compatibility with older versions.

Testing

Verified with Zabbix 6.4.21 that all item names are now found in metric
mapping (previously logged as "Item name Linux: XXX not found in metric mapping").

Impact

• No impact on users running Zabbix 6.0 or earlier
• Fixes metric collection for users running Zabbix 6.2 or later

[shin25818]

Additional Note

While this PR adds support for the Linux: prefix, I realized that Zabbix 6.2+
also introduced prefixes for other OS templates, such as:

• Windows: CPU utilization
• macOS: CPU utilization
• etc.

A more robust solution might be to normalize item names by stripping the prefix
before matching against the METRICS dictionary in check.py:

normalized_name = item_name.split(': ', 1)[-1] if ': ' in item_name else item_name
if normalized_name in METRICS:

This would handle all OS prefixes without needing to enumerate each one explicitly.
I'm happy to update this PR if the maintainer prefers this approach.

[github-actions]

This pull request has not been updated for more than 21 days. If there are no updates to this PR within 7 days, it will be closed. If you'd like to re-open this PR after it's been closed, you can start from the latest master branch or pull the latest changes into your branch and create a new pull request.

[github-actions]

This pull request was not updated after an additional 7 days of no activity. If you would like to continue work on this PR, please re-open this PR or create a fresh branch off of the latest master branch.