Reduce lock contention in SyncMetricStorage for concurrent metric recording

[perhapsmaple]

Fixes #3927

Changes

First set of changes to reduce lock contention: Move MetricAttributes construction outside the lock

Benchmark before:

Date: 2026-03-31
Build: Release, CXX14, AppleClang 17.0.0, arm64 (M-series, 8 cores)
Repetitions: 5
Threads: 4 (default)

Benchmark                                          Time (ns)     CPU (ns)    Iterations
---------------------------------------------------------------------------------------
BM_MeasurementsTest (1 thread)
  mean                                              2,237,371       12,722        10,000
  median                                            2,234,953       12,774        10,000
  stddev                                                6,464          215
  cv                                                    0.29%         1.69%

BM_MeasurementsThreadsShareCounterTest (4 threads, shared counter)
  mean                                              4,353,910       58,109         1,000
  median                                            4,471,432       59,356         1,000
  stddev                                              250,165        2,399
  cv                                                    5.75%         4.13%

BM_MeasurementsPerThreadCounterTest (4 threads, per-thread counters)
  mean                                              1,120,472       47,976        10,000
  median                                            1,122,614       49,069        10,000
  stddev                                               99,805        1,933
  cv                                                    8.91%         4.03%

Benchmark after:

Date: 2026-03-31
Build: Release, CXX14, AppleClang 17.0.0, arm64 (M-series, 8 cores)
Repetitions: 5
Threads: 4 (default)

Benchmark                                          Time (ns)     CPU (ns)    Iterations
---------------------------------------------------------------------------------------
BM_MeasurementsTest (1 thread)
  mean                                              2,230,225       11,958        10,000
  median                                            2,229,963       11,957        10,000
  stddev                                                  826         24.4
  cv                                                    0.04%         0.20%

BM_MeasurementsThreadsShareCounterTest (4 threads, shared counter)
  mean                                              2,904,767       51,929         1,000
  median                                            2,904,170       51,912         1,000
  stddev                                               17,484          800
  cv                                                    0.60%         1.54%

BM_MeasurementsPerThreadCounterTest (4 threads, per-thread counters)
  mean                                              1,031,435       45,674        10,000
  median                                            1,038,361       46,468        10,000
  stddev                                               64,472        1,719
  cv                                                    6.25%         3.76%

Shared-counter wall time dropped from 4,354us to 2,905us (33% reduction).

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (0b1c704) to head (0eef2cc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3959      +/-   ##
==========================================
+ Coverage   90.13%   90.13%   +0.01%     
==========================================
  Files         230      230              
  Lines        7293     7294       +1     
==========================================
+ Hits         6573     6574       +1     
  Misses        720      720              

Files with missing lines	Coverage Δ
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	89.19% <100.00%> (+0.62%)	⬆️

... and 1 file with indirect coverage changes

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[ThomsonTan]

The overload of GetorSetDefault which accepts KeyValueIterable could be removed with this change.

opentelemetry-cpp/sdk/include/opentelemetry/sdk/metrics/state/attributes_hashmap.h

Lines 83 to 89 in 508ec67

	Aggregation *GetOrSetDefault(
	const opentelemetry::common::KeyValueIterable &attributes,
	const AttributesProcessor *attributes_processor,
	nostd::function_ref<std::unique_ptr<Aggregation>()> aggregation_callback)
	{
	// TODO: avoid constructing MetricAttributes from KeyValueIterable for
	// hash_map_.find which is a heavy operation

[perhapsmaple]

Can the cppcheck errors be suppressed? It reports an “access after move” issue here, but this looks like a false positive.