loki_out: add stuctured_metadata_map_keys (3.2.x Backport)

[0x006EA1E5]

Resolves #9463

• Adds stuctured_metadata_map_keys config to dynamically populate stuctured_metadata from a map

This is a PR to backport #9530

Enter [N/A] in the box, if an item is not applicable to your change.

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

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

fluent/fluent-bit-docs#1527

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[0x006EA1E5]

Valgrind report

==15890== 
==15890== HEAP SUMMARY:
==15890==     in use at exit: 0 bytes in 0 blocks
==15890==   total heap usage: 9,602 allocs, 9,602 frees, 7,264,091 bytes allocated
==15890== 
==15890== All heap blocks were freed -- no leaks are possible
==15890== 
==15890== For lists of detected and suppressed errors, rerun with: -s
==15890== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)