feat: UserEvents LogExporter - add callback to control event_name

[cijothomas]

Fixes #
Design discussion issue (if applicable) #

Changes

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 36.58537% with 78 lines in your changes missing coverage. Please review.

Project coverage is 51.3%. Comparing base (83e7fae) to head (73e0e35).

Files with missing lines	Patch %	Lines
opentelemetry-user-events-logs/src/lib.rs	0.0%	62 Missing ⚠️
...entelemetry-user-events-logs/src/logs/processor.rs	74.4%	12 Missing ⚠️
...pentelemetry-user-events-logs/src/logs/exporter.rs	71.4%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #275     +/-   ##
=======================================
- Coverage   51.4%   51.3%   -0.1%     
=======================================
  Files         64      64             
  Lines       8781    8870     +89     
=======================================
+ Hits        4516    4557     +41     
- Misses      4265    4313     +48     

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[lalitb]

should it return String or Cow<'a, str>, allowing user to return the dynamic string from the callback? With current approach, the use can only use the static str, or the references to data owned by LogRecord.

[cijothomas]

@psandana also.
I think Cow would be nice, so users relying on something already from logrecord(like event name) won't have to allocate.

[utpilla]

Why use Cow? We only need &str. The user_events API does not require us to provide them an owned string.

[cijothomas]

if we use &str, then it won't be possible to generate a totally new string inside the callback and returns & to it.
We'll be restricted to returning either log record.eventname or target, but not a new string.

[utpilla]

But then we would be forcing them to always allocate a string. Let's say a user wants the result to be event_name appended by some characters, for example "123", wouldn't we be forcing them to always create a new string on the hot path? That would be bad for performance.

[cijothomas]

yes that is correct. If creating a totally new string, then it'll allocate. If using existing event_name or target (or a sub-slice of it), then it won't need allocation.

Is there an alternative to return &str from the callback? Compiler won't allow returning a &str for a newly created String inside the callback right?

[utpilla]

I think this feature should provide some mechanism to avoid allocations. For example, a user should be able to use a HashMap to cache the required mappings for event_name, target etc. and reuse those allocations for subsequent calls.

Maybe we can look at achieving this through a trait instead of a closure-based callback. The implementation of the trait could then hold any state (Hashmap etc.) and use it along with &LogRecord to return a &str. It might be possible to do it using closure-based callback as well but that might not be as straightforward.

pub trait EventNameProvider: Send + Sync {
    fn get_event_name(&self, record: &LogRecord) -> &str;
}

pub struct UserEventsExporter<EP: EventNameProvider> {
    ...
    provider: EP,
}

impl<EP: EventNameProvider> UserEventsExporter<EP> {
    pub fn new(provider: EP) -> Self {
        UserEventsExporter { ..., provider }
    }

    pub fn export(&self, record: &LogRecord) {
        ...
        let event_name = self.provider.get_event_name(record);
        ...
    }
}

pub struct MapBasedProvider {
    map: HashMap<String, String>,
}

impl EventNameProvider for MapBasedProvider {
    fn get_event_name(&self, record: &LogRecord) -> &str {
        self.map
            .get(&record.event_name)
            .map(|s| s.as_str())
            .unwrap_or("Unknown event")
    }
}