filter_log_to_metric: Add emitter alias and mem buf limit

[drbugfinder-work]

This PR will add functionality to set emitter alias and mem buf limit. The default mem buf limit is 10MB.

Needs #8473 to be applied before taking effect.
(EDIT PR is merged)

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

• [N/A] Example configuration file for the change
• [N/A] 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.

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

Documentation

• [N/A] Documentation required for this feature

Backporting

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

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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.

[drbugfinder-work]

cc @agup006

[drbugfinder-work]

root@flb:~/fluent-bit-emitter-limit/fluent-bit/build/bin# valgrind ./flb-rt-filter_log_to_metrics 
==302271== Memcheck, a memory error detector
==302271== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==302271== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==302271== Command: ./flb-rt-filter_log_to_metrics
==302271== 
Test counter_k8s...                             [ OK ]
Test counter...                                 [ OK ]
Test counter_k8s_two_tuples...                  [ OK ]
Test gauge...                                   [ OK ]
Test histogram...                               [ OK ]
Test counter_regex...                           [ OK ]
Test regex_empty_label_keys...                  [ OK ]
Test label...                                   [ OK ]
SUCCESS: All unit tests have passed.
==302271== 
==302271== HEAP SUMMARY:
==302271==     in use at exit: 0 bytes in 0 blocks
==302271==   total heap usage: 19,885 allocs, 19,885 frees, 11,297,772 bytes allocated
==302271== 
==302271== All heap blocks were freed -- no leaks are possible
==302271== 
==302271== For lists of detected and suppressed errors, rerun with: -s
==302271== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)