Support using event targets as labels

[abonander]

I had a hell of a time figuring out how to actually filter events by target in Grafana, because it appears they're not provided as labels: https://github.com/hrxi/tracing-loki/blob/master/src/lib.rs#L241

Having dug into Loki more, I understand why this choice was made (as it may exceed cardinality limits) but I would at least like to have the option.

Renaming target to _target also just makes it more annoying to use the json parser, as I have to do | json target="_target" instead of just | json target.

[hrxi]

Having dug into Loki more, I understand why this choice was made (as it may exceed cardinality limits) but I would at least like to have the option.

As you found out in the docs, it's probably something one shouldn't really do. I find it hard to justify working towards that if I don't see the use case. Can you explain why you want it after having read the docs?

[abonander]

In use-cases where the cardinality of the target field is controlled, this could be incredibly useful.

Honestly, the more flexible solution would be to support a callback that can add arbitrary labels to a single event, given its metadata.

[hrxi]

In use-cases where the cardinality of the target field is controlled, this could be incredibly useful.

And you do have such a use case, it's not entirely hypothetical?

Honestly, the more flexible solution would be to support a callback that can add arbitrary labels to a single event, given its metadata.

Currently, the code exploits that the labels never change during the runtime. Changing this assumption is probably going to cost a bit of performance. This means that there should be a clear use case.

[blastrock]

I second this issue, and would even go further. I would like to have a way to specify any field in the builder to be sent as label. Something like:

    let (layer, task) = tracing_loki::builder()
        .label("host", "mine")?
        .extra_field("pid", format!("{}", process::id()))?
        .export_field_as_label("target")
        .export_field_as_label("my_custom_field")
        .build_url(Url::parse("http://127.0.0.1:3100").unwrap())?;

    // ...

    info!(my_custom_field="one", "test"); 

Sometimes I need to filter the logs with custom fields to debug an application, and there are a lot of logs and filtering on a field is very slow. Since it's my application, I fully control the cardinality of those labels. I think this would be a very useful feature, and it lets the developer have the control on whether they want to pay the price for more indexes for faster look ups.