processor_labels: add support for record accessor in values

[edsiper]

Label processors allow to manipulate the labels from metrics, this PR extends the actions insert, update and upsert to support a record accessor pattern that helps to use the $TAG variable that can be populated dinamically, e.g:

service:
  flush: 1

pipeline:
  inputs:
    - name: event_type
      type: metrics
      tag: my_test

      processors:
        metrics:
          - name: labels
            upsert: tag $TAG

  outputs:
    - name: stdout
      match: '*'

Fluent Bit output:

______ _                  _    ______ _ _             ___  _____ 
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/ 


[2025/01/15 22:16:42] [ info] [fluent bit] version=4.0.0, commit=27734ee3e6, pid=1586419
[2025/01/15 22:16:42] [ info] [storage] ver=1.5.2, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/01/15 22:16:42] [ info] [simd    ] disabled
[2025/01/15 22:16:42] [ info] [cmetrics] version=0.9.9
[2025/01/15 22:16:42] [ info] [ctraces ] version=0.5.7
[2025/01/15 22:16:42] [ info] [input:event_type:event_type.0] initializing
[2025/01/15 22:16:42] [ info] [input:event_type:event_type.0] storage_strategy='memory' (memory only)
[2025/01/15 22:16:42] [ info] [output:stdout:stdout.0] worker #0 started
[2025/01/15 22:16:42] [ info] [input:event_type:event_type.0] thread instance initialized
[2025/01/15 22:16:42] [ info] [sp] stream processor started
[2025/01/15 22:16:43] [ info] [input:event_type:event_type.0] [OK] collector_time
2025-01-16T04:16:43.837152867Z kubernetes_network_load_counter{tag="my_test"} = 3
2025-01-16T04:16:43.837152867Z kubernetes_network_load_counter{tag="my_test",hostname="localhost",app="cmetrics"} = 1
2025-01-16T04:16:43.837152867Z kubernetes_network_load_counter{tag="my_test",hostname="localhost",app="test"} = 12.15
2025-01-16T04:16:43.837152867Z kubernetes_network_load_gauge{tag="my_test"} = 1
2025-01-16T04:16:43.837152867Z k8s_disk_load_summary{tag="my_test"} = { quantiles = { 0.1=1.1, 0.2=2.2, 0.3=3.3, 0.4=4.4, 0.5=5.5 }, sum=51.6129, count=10 }
2025-01-16T04:16:43.837152867Z k8s_disk_load_summary{tag="my_test",my_label="my_label"} = { quantiles = { 0.1=0.1, 0.2=0.2, 0.3=0.3, 0.4=0.4, 0.5=0.5 }, sum=51.6129, count=5 }
2025-01-16T04:16:43.837152867Z k8s_disk_load_summary{tag="my_test",my_label="my_val"} = { quantiles = { 0.1=11.11, 0.2=0, 0.3=33.33, 0.4=44.44, 0.5=55.55 }, sum=51.6129, count=10 }
2025-01-16T04:16:43.837152867Z k8s_network_load_histogram{tag="my_test"} = { buckets = { 0.05=2, 5=3, 10=4, +Inf=0 }, sum=1013.02, count=5 }
2025-01-16T04:16:43.837152867Z k8s_network_load_histogram{tag="my_test",my_label="my_val"} = { buckets = { 0.05=2, 5=3, 10=4, +Inf=0 }, sum=1013.02, count=5 }

This PR also adds a new unit test for the processor.

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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.

[cosmo0920]

This is just my curiosity but do we need to use this if def for test cases? This is because when disabling FLB_HAVE_METRICS, the defined test cases are not effective under this condition.

[edsiper]

hmm maybe we don't need it... I don't think metrics are optional anymore