cmt_math: fix metric conversions

[aidanleuck]

Fixed issue with metric conversions where garbage values were being returned from a union. This method now implicitly does a cast as seemed to be intended by the original method. This issue was found because I was having an issue where fluent bit was spitting out garbage values for my metric counters when using the opentelemetry_input function.

Fixes #8083

• Example configuration file for the change
• Debug log output from testing the change

• [ N/A ] Attached Valgrind output that shows no leaks or memory corruption was found
• 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

• [ 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.

[aidanleuck]

Example Configuration with change

[SERVICE]
    Flush        1
    Daemon       Off
    Log_level    debug
    Http_Server  On
    Config_Watch On
    Parsers_File parsers.conf
    Parsers_File custom_parsers.conf
    Health_Check Off
[INPUT]
    name opentelemetry
    listen 0.0.0.0
    port 4318
    successful_response_code 200
    tag otel
[OUTPUT]
    Name stdout
    Match otel

[aidanleuck]

Debug output for change:
Before:

[2024/04/25 16:01:45] [debug] [task] created task=0x7ffff0048c70 id=0 OK
[2024/04/25 16:01:45] [debug] [output:stdout:stdout.0] task_id=0 assigned to thread #0
2024-04-25T16:01:45.653451200Z _Weather_Temperature{summary="Warm"} = 2.4703282292062327e-322
2024-04-25T16:01:45.653451200Z _Weather_Temperature{summary="Cool"} = 4.001931731314097e-322
2024-04-25T16:01:45.653451200Z _Weather_Temperature{summary="Scorching"} = 6.4228533959362051e-323
2024-04-25T16:01:45.653451200Z _Weather_Temperature{summary="Bracing"} = 2.1738888417014848e-322
[2024/04/25 16:01:45] [debug] [out flush] cb_destroy coro_id=0
[2024/04/25 16:01:45] [debug] [task] destroy task=0x7ffff0048c70 (task_id=0)

After:

[2024/04/25 15:54:45] [debug] [output:stdout:stdout.0] task_id=0 assigned to thread #0
2024-04-25T15:54:45.653401200Z _Weather_Temperature{summary="Warm"} = 50
2024-04-25T15:54:45.653401200Z _Weather_Temperature{summary="Cool"} = 81
2024-04-25T15:54:45.653401200Z _Weather_Temperature{summary="Scorching"} = 13
2024-04-25T15:54:45.653401200Z _Weather_Temperature{summary="Bracing"} = 44
[2024/04/25 15:54:45] [debug] [out flush] cb_destroy coro_id=26
[2024/04/25 15:54:45] [debug] [task] destroy task=0x7ffff0047c40 (task_id=0)

[cosmo0920]

Hi, thanks for fixing this!

However, we're tracking down for cmetrics' issues in https://github.com/fluent/cmetrics.
Could you send your patch there instead of this repo?

[edsiper]

@aidanleuck thanks for triagging and proposing a solution for this issue.

Some context: for fixes that lives in lib/, they should be handled in a separate repository and then we sync up the project here in this one.

To speed up things, I noticed the code I original wrote uses an union when is not needed since we can do a direct cast, not sure why the code suggested here works but I suspect it can be something around how the compiler is working differently behind the scenes.

With the intention to speed up a fix and provide a cleaner version with also limit checks, I have submitted this PR:

fluent/cmetrics#204

My hope is that the issue goes away, but if still persist we will need to investigate more.

[aidanleuck]

@edsiper Thank you for opening up that PR in the lib repository, I appreciate it! I will close this pull request.