logs_to_metrics: Support optional value_field for counters

[databus23]

Support incrementing counters by a specified value

If a value_field is specified for a counter metric, the counter is incremented by the given value instead of just incrementing by 1.

This allows counting more things then just the number of records, such as the total number of bytes sent or received from an access log.

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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
  See docs PR for a configuration example: filter_logs_to_metrics: document field_value usage for counters fluent-bit-docs#1584
• [N/A] Debug log output from testing the change

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

Documentation

• Documentation required for this feature

Doc PR: fluent/fluent-bit-docs#1584

Backporting

• 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.

[databus23]

Valgrind output for unit tests:

fluent-bit ➜ /workspaces/fluent-bit/build (logs_to_metric_counter_value) $ valgrind bin/flb-rt-filter_log_
to_metrics 
==35868== Memcheck, a memory error detector
==35868== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==35868== Using Valgrind-3.19.0 and LibVEX; rerun with -h for copyright info
==35868== Command: bin/flb-rt-filter_log_to_metrics
==35868== 
Test counter_k8s...                             [ OK ]
Test counter...                                 [ OK ]
Test counter_value_field...                     [ 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.
==35868== 
==35868== HEAP SUMMARY:
==35868==     in use at exit: 0 bytes in 0 blocks
==35868==   total heap usage: 22,585 allocs, 22,585 frees, 16,092,238 bytes allocated
==35868== 
==35868== All heap blocks were freed -- no leaks are possible
==35868== 
==35868== For lists of detected and suppressed errors, rerun with: -s
==35868== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)