perf(sdk,otlp): add SpanBatch for borrowed span export, eliminate exporter clones

[bryantbiggs]

Summary

Changes SpanExporter::export from Vec<SpanData> to SpanBatch<'_> — a thin borrowed wrapper around &[SpanData]. This matches the LogBatch<'_> pattern already used in the logs pipeline. Additionally eliminates unnecessary SpanData deep clones in the tonic and HTTP trace exporters.

This is a breaking change to the SpanExporter trait. Implementors need to update their export signature and use batch.iter() or batch.as_slice() to access spans.

Supersedes #3066. Related to #3368.

Motivation

The span export path currently requires ownership transfer of a Vec<SpanData>, which forces unnecessary allocations and clones:

1. batch.split_off(0) in BatchSpanProcessor — allocates a new Vec every export cycle just to hand ownership to the exporter, even though the exporter only needs to read the spans
2. vec![span] in SimpleSpanProcessor — heap-allocates a Vec for a single span
3. span_data.clone().into() in group_spans_by_resource_and_scope — deep-clones every SpanData during proto conversion because the function consumed the Vec
4. batch.iter().cloned().collect() in tonic trace exporter — deep-clones every SpanData into an Arc<Vec<_>> for the retry closure, even though retries rarely happen
5. spans.iter().cloned().collect() in HTTP trace exporter — deep-clones spans into an intermediate Vec before proto conversion, serving no purpose

The logs pipeline already solved this with LogBatch<'_>. There's even a TODO in the codebase (span_processor.rs):

"TODO: Compared to Logs, this requires new allocation for vec for every export. See if this can be optimized by not requiring ownership in the exporter."

Allocation impact (512-span batch via tonic)

Metric	Before	After	Reduction
Vec allocations per export cycle	1 (split_off)	0 (buffer reuse)	100%
SpanData deep clones (proto conversion)	512	0	100%
SpanData deep clones (tonic retry Arc)	512	0	100%
SpanData deep clones (HTTP intermediate Vec)	512	0	100%
Estimated heap allocs per batch	~28,000–38,000	~10,000–13,000	~40–50%

Ecosystem alignment

Project	Export interface	Pattern
Go OTel SDK	[]ReadOnlySpan (borrowed slice)	Borrowed
Java OTel SDK	Collection<SpanData> (immutable)	Immutable
Rust OTel (logs)	LogBatch<'_> (borrowed)	Borrowed
Rust OTel (traces, before)	Vec<SpanData> (owned)	Consuming
Rust OTel (traces, after)	SpanBatch<'_> (borrowed)	Borrowed

What changed

opentelemetry-sdk (core change)

• New SpanBatch<'a> type wrapping &'a [SpanData] with new(), iter(), and as_slice() methods
• SpanExporter::export signature: Vec<SpanData> → SpanBatch<'_>
• BatchSpanProcessor: replaced split_off(0) with SpanBatch::new(&batch) + batch.clear()
• SimpleSpanProcessor: replaced vec![span] with stack array + SpanBatch::new(&spans)
• Updated InMemorySpanExporter, test exporters, and benchmarks

opentelemetry-proto

• Added From<&SpanData> for Span and From<&Link> for span::Link (borrowing conversions)
• group_spans_by_resource_and_scope now takes &[SpanData] instead of Vec<SpanData>

opentelemetry-otlp

• Tonic trace exporter: replaced Arc<Vec<SpanData>> (512 deep clones) with Arc<SpanBatch<'_>> (~free), matching the existing Arc<LogBatch> pattern in the tonic logs exporter
• HTTP trace exporter: removed unnecessary intermediate Vec<SpanData> clone, using spans.as_slice() directly

opentelemetry-stdout, opentelemetry-zipkin

• Updated trace exporters to accept SpanBatch<'_>

Migration guide

// Before
impl SpanExporter for MyExporter {
    async fn export(&self, batch: Vec<SpanData>) -> OTelSdkResult {
        for span in &batch { /* ... */ }
        Ok(())
    }
}

// After
use opentelemetry_sdk::trace::SpanBatch;

impl SpanExporter for MyExporter {
    async fn export(&self, batch: SpanBatch<'_>) -> OTelSdkResult {
        for span in batch.iter() { /* ... */ }
        Ok(())
    }
}

Verification

cargo fmt --all -- --check                    # clean
cargo clippy --workspace --all-features       # zero warnings
RUSTFLAGS="-D warnings" cargo check --workspace --tests  # compiles with deny(warnings)
cargo test --workspace                        # all pass, zero failures

[codecov]

Codecov Report

❌ Patch coverage is 69.44444% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.2%. Comparing base (9650783) to head (817cf54).

Files with missing lines	Patch %	Lines
opentelemetry-proto/src/transform/trace.rs	63.6%	20 Missing ⚠️
opentelemetry-stdout/src/trace/exporter.rs	0.0%	5 Missing ⚠️
opentelemetry-zipkin/src/exporter/mod.rs	0.0%	4 Missing ⚠️
opentelemetry-otlp/src/exporter/tonic/trace.rs	0.0%	2 Missing ⚠️
opentelemetry-otlp/src/exporter/http/trace.rs	0.0%	1 Missing ⚠️
opentelemetry-otlp/src/span.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3379     +/-   ##
=======================================
- Coverage   83.2%   83.2%   -0.1%     
=======================================
  Files        128     128             
  Lines      25045   25112     +67     
=======================================
+ Hits       20858   20905     +47     
- Misses      4187    4207     +20     

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