sdk/trace: apply AttributeValueLengthLimit to attribute.MAP

[pellared]

Fixes #7956

Per the OTel spec, attribute value limits must be applied recursively to map values. Previously truncateAttr handled STRING, STRINGSLICE, BYTESLICE, and SLICE, but not MAP.

Add MAP handling to the trace SDK attribute limiter:

• truncateAttr: truncates attribute.MAP values when needed.
• truncateValue: recursively truncates MAP, SLICE, STRINGSLICE, STRING, and BYTESLICE values.
• needsTruncation: pre-scan guard for MAP values to avoid rebuilding when no nested value changes.

benchstat for BenchmarkSpanLimits/AttributeValueLengthLimit (count=10):

SpanLimits/AttributeValueLengthLimit-20   14.88µ ± 16%   15.56µ ± 53%       ~ (p=0.739 n=10)
SpanLimits/AttributeValueLengthLimit-20   12.71Ki ± 0%   13.66Ki ± 0%   +7.52% (p=0.000 n=10)
SpanLimits/AttributeValueLengthLimit-20    47.00 ± 0%     56.00 ± 0%   +19.15% (p=0.000 n=10)

The extra allocations are expected in the truncation path because truncated MAP values are rebuilt recursively.

benchstat for BenchmarkSpanLimits/AttributeValueLengthLimit (count=10):

SpanLimits/None-20   14.18µ ± 7%   12.86µ ± 18%   -9.31% (p=0.043 n=10)
SpanLimits/None-20   12.16Ki ± 0%  12.16Ki ± 0%       ~ (p=1.000 n=10)
SpanLimits/None-20   38.00 ± 0%    38.00 ± 0%          ~ (p=1.000 n

No extra allocations where nothing needs to be rebuild.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.5%. Comparing base (6e2b921) to head (80f1b72).

Files with missing lines	Patch %	Lines
sdk/trace/span.go	90.9%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8454   +/-   ##
=====================================
  Coverage   83.4%   83.5%           
=====================================
  Files        322     322           
  Lines      25645   25667   +22     
=====================================
+ Hits       21410   21434   +24     
+ Misses      3853    3852    -1     
+ Partials     382     381    -1     

Files with missing lines	Coverage Δ
sdk/trace/span_limits.go	100.0% <ø> (ø)
sdk/trace/span.go	87.4% <90.9%> (+0.1%)	⬆️

... and 2 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR updates the trace SDK’s attribute value truncation logic to comply with the OpenTelemetry spec requirement that attribute value length limits be applied recursively to attribute.MAP values (and nested values within maps).

Changes:

• Extend truncateAttr, truncateValue, and needsTruncation in sdk/trace/span.go to recursively truncate attribute.MAP values while avoiding rebuilds when no nested value changes.
• Add unit tests covering truncation behavior for MAP values, including nested MAP/SLICE combinations and UTF-8 edge cases.
• Update SpanLimits documentation, benchmarks, and the changelog to reflect MAP support.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sdk/trace/span.go	Adds recursive truncation and pre-scan guard logic for attribute.MAP values.
sdk/trace/span_test.go	Adds comprehensive test cases validating MAP truncation behavior (including nested structures).
sdk/trace/span_limits.go	Updates AttributeValueLengthLimit documentation to include MAP semantics.
sdk/trace/benchmark_test.go	Includes MAP attributes in the SpanLimits benchmarks to exercise the new truncation path.
CHANGELOG.md	Documents recursive AttributeValueLengthLimit application for attribute.MAP in the trace SDK.