in_node_exporter_metrics: Fix registering callback for systemd metrics

[cosmo0920]

Fixed for registering callback settings for systemd metrics.

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

• 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

fluent-bit starts with: valgrind bin/fluent-bit -i node_exporter_metrics -p metrics=systemd -o stdout

==85698== 
==85698== HEAP SUMMARY:
==85698==     in use at exit: 0 bytes in 0 blocks
==85698==   total heap usage: 128,161 allocs, 128,161 frees, 6,672,526,968 bytes allocated
==85698== 
==85698== All heap blocks were freed -- no leaks are possible
==85698== 
==85698== For lists of detected and suppressed errors, rerun with: -s
==85698== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• 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

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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