[CODE HEALTH] Move SDK trace and metrics test helpers into anonymous namespaces

[thc1006]

Part of #4196

This is the last of three batches after #4301 and #4302, and clears the SDK test sites in #4196.

file	classes
sdk/test/metrics/exemplar/aligned_histogram_bucket_exemplar_reservoir_test.cc	AlignedHistogramBucketExemplarReservoirTestPeer
sdk/test/metrics/measurements_benchmark.cc	MockMetricExporter
sdk/test/trace/batch_span_processor_test.cc	MockSpanExporter, BatchSpanProcessorTestPeer, MockLogHandler
sdk/test/trace/tracer_provider_set_test.cc	TestProvider
sdk/test/trace/tracer_test.cc	MockSampler, MockDecisionSampler, MockIdGenerator

Shape of the change

tracer_test.cc and measurements_benchmark.cc already had an anonymous namespace that stopped just short of the flagged classes, so those are extended rather than joined by a second one.

batch_span_processor_test.cc is the one worth a second look. It already sits inside a named namespace opened by OPENTELEMETRY_BEGIN_NAMESPACE, so the anonymous namespace nests inside that and closes just before OPENTELEMETRY_END_NAMESPACE. It covers every class and every TEST in the file, including the BatchSpanProcessorOptionsEnvTest group at the end.

tracer_provider_set_test.cc closes its namespace after the trailing #endif of the NO_GETENV block, so the placement holds whether or not that block is compiled.

Warning limit

151 to 142 and 161 to 152, nine on each preset since none are ABI gated.

Cut from main rather than stacked on #4301 and #4302. If any of the three merge before this, the limit needs recomputing down by the merged batch's count. Happy to rebase whenever; they were kept independent so they can be reviewed in any order.

Verification

• All nine classes end up inside an anonymous namespace, checked by finding the nearest namespace open and close around each rather than by eye.
• Every file compiles under -DOPENTELEMETRY_ABI_VERSION_NO=1 and =2.
• BatchSpanProcessorTestPeer and AlignedHistogramBucketExemplarReservoirTestPeer are the only TEST_F suite names in the batch, and each appears in exactly one file, so no all_tests redefinition. This was the specific risk from [CODE HEALTH] Move api/test and sdk/test classes into anonymous namespace #4217 and the main thing I wanted to rule out.
• measurements_benchmark.cc is a benchmark target, so it needs to link rather than only compile; the change is a namespace wrap and does not touch linkage of any symbol the benchmark harness needs.
• clang-format 18 clean.

Same honest gap as the earlier two batches: misc-use-internal-linkage only reports classes from llvm-22 onward, so a local clang-tidy run returns zero regardless and proves nothing. The site list is from the CI log artifact for main, and CI is the authority on the count.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.24%. Comparing base (cf62199) to head (4a57b6e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4303      +/-   ##
==========================================
- Coverage   81.25%   81.24%   -0.01%     
==========================================
  Files         446      446              
  Lines       18872    18872              
==========================================
- Hits        15332    15330       -2     
- Misses       3540     3542       +2     

see 1 file with indirect coverage changes

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[marcalff]

LGTM