Provide ability to define span.kind for OpenTelemetry Layer

[alessandrobologna]

Hi,
I wanted to propose a small enhancement to the OpenTelemetry tracing layer implementation in the AWS Lambda Rust runtime.

Currently, in otel.rs, the tracing span for Lambda invocations doesn't include a way to specify the span kind (e.g., server, client). Adding this capability could improve how systems are represented in observability platforms.

My suggestion: we could add an otel.kind field to the span creation, initialized as empty. This would allow users to set the span kind in their handlers if needed. For example:

let span = tracing::info_span!(
    "Lambda function invocation",
    "otel.name" = req.context.env_config.function_name,
    "otel.kind" = field::Empty,  // New field for span kind
    { traceconv::FAAS_TRIGGER } = &self.otel_attribute_trigger,
    { traceconv::FAAS_INVOCATION_ID } = req.context.request_id,
    { traceconv::FAAS_COLDSTART } = self.coldstart
);

Users could then set the span kind in their handler like this:

use tracing::Span;

async fn handler(event: Value, _: Context) -> Result<Value, Error> {
    let span = Span::current();
    span.record("otel.kind", "server");
    
    // Handler logic...
}

This change should be backward compatible and provide more flexibility for users. I'd love to hear your thoughts on this, and happy to propose a PR!

[maxday]

closing this as #946 as been merged.

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