output: Fix handling of metrics in output processor

[tchrono]

This patch passes an cmt_out_context pointer so that output processors can modify metrics.

Without this change, it is not possible to modify metrics in an output processor. Example config which reproduces:

service:
    flush: 1
    log_level: info

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

    outputs:
        - name: stdout
          match: event
          processors:
            metrics:
              - name: metrics_selector
                metric_name: /kubernetes/
                action: include

Without the patch, we observe unmodified metrics:

2024-05-21T17:51:30.526411118Z kubernetes_network_load_counter = 3
2024-05-21T17:51:30.526411118Z kubernetes_network_load_counter{hostname="localhost",app="cmetrics"} = 1
2024-05-21T17:51:30.526411118Z kubernetes_network_load_counter{hostname="localhost",app="test"} = 12.15
2024-05-21T17:51:30.526411118Z kubernetes_network_load_gauge = 1
2024-05-21T17:51:30.526411118Z k8s_disk_load_summary = { 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 }
2024-05-21T17:51:30.526411118Z k8s_disk_load_summary{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 }
2024-05-21T17:51:30.526411118Z k8s_network_load_histogram = { buckets = { 0.05=2, 5=3, 10=4, +Inf=5 }, sum=1013.02, count=5 }
2024-05-21T17:51:30.526411118Z k8s_network_load_histogram{my_label="my_val"} = { buckets = { 0.05=2, 5=3, 10=4, +Inf=5 }, sum=1013.02, count=5 }

With the patch, we see the filtered metrics:

2024-05-21T17:52:29.525856283Z kubernetes_network_load_counter = 3
2024-05-21T17:52:29.525856283Z kubernetes_network_load_counter{hostname="localhost",app="cmetrics"} = 1
2024-05-21T17:52:29.525856283Z kubernetes_network_load_counter{hostname="localhost",app="test"} = 12.15
2024-05-21T17:52:29.525856283Z kubernetes_network_load_gauge = 1

valgrind output:

==117643== Memcheck, a memory error detector
==117643== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==117643== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==117643== Command: ./build/bin/fluent-bit -c metrics-processor.yaml
==117643== Parent PID: 12681
==117643== 
==117643== 
==117643== HEAP SUMMARY:
==117643==     in use at exit: 0 bytes in 0 blocks
==117643==   total heap usage: 4,189 allocs, 4,189 frees, 2,700,236 bytes allocated
==117643== 
==117643== All heap blocks were freed -- no leaks are possible
==117643== 
==117643== For lists of detected and suppressed errors, rerun with: -s
==117643== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

---

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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

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.

---

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]

Looks good. And I'd overlooked for output side of processor functionality.
Thanks for tackling this.
Is there any chance to add unit tests for output patterns like as following file?
https://github.com/fluent/fluent-bit/blob/master/tests/runtime/processor_metrics_selector.c

[tchrono]

@cosmo0920 added a runtime test that attaches the processor to output.

[edsiper]

@tchrono @tarruda

Please adjust the commit message Add runtime test for output modifying metrics, it needs to be prefixed with the interface, e.g:

• tests: runtime: processor_metrics_selector: ....

[cosmo0920]

The added commit is OK. But it needs to fix commit message.

[tchrono]

@edsiper @cosmo0920 fixed commit message

[cosmo0920]

I approved for the workflows. Waiting for the results.

[cosmo0920]

Thanks for tackling this. Looks good to me.