filter_log_to_metrics: Fix config key after rename

[wndhydrnt]

After #7739 was merged, the config key label was renamed to add_label in #7964.

Not all occurrences of the key label were renamed in #7964, which leads to a regression and failing tests.

This change renames the key throughout the code of the filter.

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Testing
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• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

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• [N/A] Backport to latest stable release.

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[wndhydrnt]

Configuration:

[SERVICE]
    Flush     1
    Log_Level debug

[INPUT]
    Name  dummy
    Dummy {"message":"This is a test","kubernetes":{"labels":{"app":"testservice"}}}

[FILTER]
    Name               log_to_metrics
    Match              *
    Tag                test_metric
    Add_label          app $kubernetes['labels']['app']
    Metric_mode        counter
    Metric_name        count_all_dummy_messages
    Metric_description This metric counts dummy messages

[OUTPUT]
    Name  stdout
    Match *

[OUTPUT]
    Name  prometheus_exporter
    Match test_metric
    Host  0.0.0.0
    Port  2021

[pwhelan]

It seems like I merged part of your change in #7972. Mind rebasing off of master so we can get the fixed log error message?

[wndhydrnt]

Thanks for the hint @pwhelan. Now this PR contains a change to a log message only.

@cosmo0920 Feel free tom erge or close this MR as you see fit.

[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[wndhydrnt]

Closing due to inactivity.