in_node_exporter_metrics: handle not as error when missing cpu_throttle_metrics

[cosmo0920]

node_cpu_core_throttles_total and node_cpu_package_throttles_total metrics are not existing in the Virtual Machine (VM) environment,
In VM environment, they are just missing.
So, when they are absent, we should handle to output this issue with debug messages and not to handle as error.
The official node_exporter uses the same approach:
https://github.com/prometheus/node_exporter/blob/master/collector/cpu_linux.go#L267-L287

Fixes #7589.

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

$ bin/fluent-bit -i node_exporter_metrics -pmetrics=cpu -o stdout

• Debug log output from testing the change

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

==564825== 
==564825== HEAP SUMMARY:
==564825==     in use at exit: 95,170 bytes in 702 blocks
==564825==   total heap usage: 6,896 allocs, 6,194 frees, 11,082,193 bytes allocated
==564825== 
==564825== LEAK SUMMARY:
==564825==    definitely lost: 0 bytes in 0 blocks
==564825==    indirectly lost: 0 bytes in 0 blocks
==564825==      possibly lost: 0 bytes in 0 blocks
==564825==    still reachable: 95,170 bytes in 702 blocks
==564825==         suppressed: 0 bytes in 0 blocks
==564825== Reachable blocks (those to which a pointer was found) are not shown.
==564825== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==564825== 
==564825== For lists of detected and suppressed errors, rerun with: -s
==564825== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

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Backporting

• Backport to latest stable release.

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