feat(sdk): support OTEL_ATTRIBUTE_COUNT_LIMIT env var

[Wassbdr]

Part of #3374

Problem

OTEL_ATTRIBUTE_COUNT_LIMIT is the spec-defined general fallback for the maximum span attribute count (SDK configuration spec). The SDK only read the span-specific OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT, so setting the general variable had no effect. This is one of the gaps tracked in #3374.

Change

In Config::default(), read OTEL_ATTRIBUTE_COUNT_LIMIT and apply it to max_attributes_per_span, then let OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT override it when both are set (per the spec's precedence). Code-based configuration (e.g. with_max_attributes_per_span) continues to take precedence over both, since the builder starts from Config::default().

The general limit deliberately does not touch the per-event / per-link attribute count limits — those have their own spec defaults (128) and are not governed by OTEL_ATTRIBUTE_COUNT_LIMIT.

Tests

• otel_attribute_count_limit_sets_span_attribute_limit — general var applies to span attributes and leaves per-event / per-link limits at their defaults.
• span_attribute_count_limit_overrides_general_limit — span-specific var wins when both are set.
• default_span_attribute_limit_when_unset — default of 128 when neither is set.

Also documented the variable and its precedence in the crate-level env-var table and added a CHANGELOG entry.

cargo test -p opentelemetry_sdk --all-features --lib trace::config, cargo clippy --all-targets --all-features -- -Dwarnings, and cargo fmt --all -- --check all pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.0%. Comparing base (1b3846c) to head (ef52166).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3568   +/-   ##
=====================================
  Coverage   82.9%   83.0%           
=====================================
  Files        130     130           
  Lines      27768   27809   +41     
=====================================
+ Hits       23040   23082   +42     
+ Misses      4728    4727    -1     

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[cijothomas]

OTEL_ATTRIBUTE_COUNT_LIMIT -> is this supposed to affect spans and logs too?

[cijothomas]

I think we should do this only after Logs also support this, else this may misled users to believing that setting this ENV variable will enforce limits, but in reality - Logs don't enforce it.

[cijothomas]

Thanks for your contribution. Please see https://github.com/open-telemetry/opentelemetry-rust/pull/3568/changes#r3471676694

This is better done after Logging fixes.

[Wassbdr]

Good point — thanks. The logs SDK doesn't enforce any attribute limit today, so I've opened #3571 to add that first (OTEL_LOGRECORD_ATTRIBUTE_COUNT_LIMIT + with_max_attributes_per_log, with OTEL_ATTRIBUTE_COUNT_LIMIT as fallback, enforced in SdkLogger::emit).

Once #3571 lands, I'll update this PR so OTEL_ATTRIBUTE_COUNT_LIMIT consistently acts as the general fallback for both spans and logs. Happy to keep this one open and rebase on top of #3571, or close it and fold the general-fallback wording into a follow-up — whichever you prefer.