log_to_metric tags converted to lowercase

[GGonzalezGomez]

Hi!

I'm not sure if this is a bug, working as expected or simply bad configuration by my side.
I'm going through the log_to_metric transform which allows to set the metric labels/tags. According to documentation you can use "*" to add a log field map with tags. I've used it but for some reason the tag keys are converted to lower case. Only for those added with "*".

In the example below, I added the field tags which has a key named LogGroupName. On the metric tags that key is converted to lower case loggroupname. But, if I add a tag directly with upper case, Aws_account_id, it is left without changes.

type: "log_to_metric"
    inputs:
    - "inputstep"
    metrics:
    - type: "counter"
      ............
      tags:
        "Aws_account_id": "xxxxxxxxxxx"
        "environment": "development"
        "*": "{{ tags }}"

Log input:
{"count":16.0,"dimensions":{"LogGroupName":"example-log-group"},"max":3.0,"metric_name":"IncomingLogEvents", "min":1.0,"namespace":"AWS/Logs","region":"eu-west-1","sum":20.0,"tags":{"LogGroupName":"example-log-group"},"timestamp":"2025-02-27T15:54:00Z","unit":"None"}

Metric output:
{"name":"incominglogevents_sum","tags":{"Aws_account_id":"xxxxxxxxxxx","environment":"development","loggroupname":"example-log-group"},"timestamp":"2025-02-27T15:54:00Z","kind":"absolute","counter":{"value":20.0}}

Thanks!

[pront]

Hi @GGonzalezGomez, I verified this on my side. It boils down to this line: https://github.com/vectordotdev/vector/blame/6fe2317d0b1bbc477fafd9ff80f39de55d36dffc/src/common/expansion.rs#L28

Do you want to a feature request to preserve the case when expanding tags? Ideally that would be the default but it's preferred to not break existing behavior.

[GGonzalezGomez]

Hi @pront, that's ok, existing behavior is fine for me. It just looks a little bit random to convert it to lowercase, but probably there's a use case that makes sense for that.

Thanks for checking!