Merge :sdk:metrics-testing into :sdk:testing

[jack-berg]

With exponential histogram stable, there's nothing left in :sdk:metrics-testing so it can be deleted.

[codecov]

Codecov Report

Base: 91.11% // Head: 91.14% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (857e5c8) compared to base (c427bf2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5144      +/-   ##
============================================
+ Coverage     91.11%   91.14%   +0.03%     
- Complexity     4905     4933      +28     
============================================
  Files           547      551       +4     
  Lines         14503    14576      +73     
  Branches       1393     1396       +3     
============================================
+ Hits          13214    13286      +72     
- Misses          891      894       +3     
+ Partials        398      396       -2     

Impacted Files	Coverage Δ
...l/data/ImmutableExponentialHistogramPointData.java	100.00% <ø> (ø)
...ing/assertj/ExponentialHistogramBucketsAssert.java	100.00% <ø> (ø)
...nal/data/ImmutableExponentialHistogramBuckets.java	100.00% <100.00%> (ø)
...dk/testing/assertj/ExponentialHistogramAssert.java	100.00% <100.00%> (ø)
...sting/assertj/ExponentialHistogramPointAssert.java	100.00% <100.00%> (ø)
...pentelemetry/sdk/testing/assertj/MetricAssert.java	95.65% <100.00%> (+0.41%)	⬆️
...gregator/DoubleExponentialHistogramAggregator.java	93.33% <0.00%> (-3.34%)	⬇️
...metry/sdk/metrics/export/PeriodicMetricReader.java	90.00% <0.00%> (+2.85%)	⬆️
... and 1 more

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