[Feature]: Reading spans through the public API requires cloning everything to SpanData

[paullegranddc]

Related Problems?

No response

Describe the solution you'd like:

When creating a new span, the tracer calls the SpanProcessor::on_start on method and passes a &mut Span.
In the otel spec, this span parameter is read/write opentelemetry-specification.

Currently in the rust SDK, the only way to read data on the span is to call span.exported_data() -> Option<crate::trace::SpanData>, which clones all fields in the span, including things that require allocation such as the span links attributes, events and span links.

Other opentelemetry SDK implementation usually expose a ReadableSpan interface
go java providing a view over the span without mutations, and I believe something similar could be implemented over the rust Span.

This is the API I am proposing:

impl Span {
    /// Returns a read-only view of the span data
    /// This is None if the span is not recording.
    pub fn read<'a>(&'a self) -> Option<ReadableSpan<'a>> {
        Some(ReadableSpan(self.data.as_ref()?))
    }
}

#[derive(Clone, Debug)]
pub struct ReadableSpan<'a>(&'a SpanData);

impl<'a> ReadableSpan<'a> {
    pub fn parent_span_id(&self) -> SpanId {..}
    pub fn span_kind(&self) -> SpanKind {..}

    pub fn name(&self) -> &str {..}

    pub fn start_time(&self) -> SystemTime {..}

    pub fn end_time(&self) -> SystemTime {..}

    pub fn attributes(&self) -> &'a [KeyValue] {..}

    pub fn dropped_attributes_count(&self) -> u32 {..}

    pub fn events(&self) -> &'a [Event] {..}

    pub fn dropped_events_count(&self) -> u32 {..}

    pub fn links(&self) -> &'a [Link] {..}

    pub fn dropped_links_count(&self) -> u32 {..}

    pub fn status(&self) -> &Status {..}
}

POC here

Considered Alternatives

I see two alternatives:

1. An alternative would be to implement all of the getters directly on the opentelemetry_sdk::trace::Span struct.
   This is functionally the same, and we don't have to add a separate struct to the public API of ths crate.
   This make the API a bit harder to call though because every field that references a field in SpanData would need to either return an Option, or the default value (empty attribute slice, 0 for dropped entries, SpanId::Invalid for the parent id)

2. Make ReadableSpan<'a> a trait, and return an opaque object implementating the trait from Span

impl Span {
    /// Returns a read-only view of the span data
    /// This is None if the span is not recording.
    pub fn read<'a>(&'a self) -> Option<impl ReadableSpan<'a>> {
        Some(ReadableSpan(self.data.as_ref()?))
    }
}


trait ReadableSpan<'a> {...}

This is more complex, as the value become un-nameable and thus cannot be stored easily in collections.
I don't really know what this buys us, and who else would implement the trait....

Additional Context

No response