feat(sdk): support OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT

[bryantbiggs]

Summary

Implements OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT per the SDK environment variable specification. When configured, Value::String and Array::String attribute values are truncated to the specified number of Unicode characters on spans, span events, and span links.

This is a spec-required feature that Go and Java SDKs have supported since their 1.0 releases. Its absence is tracked as part of #3374 (missing spec-required environment variables). The stale PR #3090 by @msierks-pcln attempted this 6+ months ago but stalled after CLA issues; this implementation incorporates the maintainer review feedback from that PR.

Supersedes #3090.

Why this belongs in a single PR

The changes span two crates but form a single logical unit:

1. opentelemetry crate — Value::truncate() and StringValue::truncate() are general-purpose string truncation methods needed because StringValue's internal storage is pub(crate), so the SDK crate cannot manipulate it directly. These methods contain no enforcement logic — they are utilities.

2. opentelemetry-sdk crate — The enforcement: reading the env var, storing the limit in SpanLimits, and calling truncate() at every attribute ingestion point (span build, set_attribute, add_event, add_link).

Splitting these into separate PRs would leave the API methods without a caller, or the SDK enforcement without the methods it calls.

Design decisions

• u32 not i32 — only positive values are meaningful (addresses feat: add support for OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT #3090 reviewer feedback)
• Option<u32> with None default — no sentinel needed; unlimited by default per spec
• Only OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT, not OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT — the global fallback should wait until logs also support it (per maintainer feedback on feat: add support for OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT #3090: "OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT is not something we can add right now, as Logs don't respect this at all"). This is consistent with how OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT exists without a global OTEL_ATTRIBUTE_COUNT_LIMIT fallback in this codebase.
• Truncation is silent — no ellipsis appended, matching spec behavior (addresses feat: add support for OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT #3090 reviewer feedback about being explicit about implications)
• Truncation respects UTF-8 character boundaries — uses char_indices().nth() to find the byte offset, never splits a multi-byte character

Breaking change

SpanLimits gains a new public field max_attribute_value_length: Option<u32>. Code that constructs SpanLimits via struct literal will need to add this field (use None for unlimited). This is a known limitation of SpanLimits not being #[non_exhaustive], tracked in #2551.

Changes

• opentelemetry: add Value::truncate() and StringValue::truncate() with 8 unit tests
• SpanLimits: add max_attribute_value_length: Option<u32> field and truncate_string_values() helper
• Config: read OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT env var with 4 unit tests
• TracerProviderBuilder: add with_max_attribute_value_length(u32) builder method
• Span / SdkTracer: apply truncation at all attribute ingestion points with 5 integration tests

Test plan

• StringValue::truncate: ASCII, empty, zero-length, multi-byte UTF-8 (euro sign, CJK, accented chars)
• Value::truncate: only affects String/Array::String, leaves bool/i64/f64 and their arrays unchanged
• Span attribute truncation via builder and set_attribute
• UTF-8 boundary correctness
• Event attribute truncation
• Link attribute truncation
• No truncation when limit is not set
• Env var parsing: valid, invalid, negative
• cargo fmt --all -- --check
• cargo clippy (both crates)
• bash ./scripts/lint.sh
• Full workspace cargo check
• 85 trace tests pass, 13 common tests pass

[codecov]

Codecov Report

❌ Patch coverage is 96.36364% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.4%. Comparing base (9650783) to head (a37a513).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/trace/span.rs	92.4%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3377     +/-   ##
=======================================
+ Coverage   83.2%   83.4%   +0.1%     
=======================================
  Files        128     128             
  Lines      25045   25319    +274     
=======================================
+ Hits       20858   21123    +265     
- Misses      4187    4196      +9     

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