Added DelegatingMetricData & TestMetricData Classes

[ThomasJazz]

Hello! This is my first attempt to contribute to an open source project, so pls be nice :). I have read through the contributor steps, but I apologize in advance if I have missed anything.

• Changes should address issue: https://github.com/open-telemetry/opentelemetry-java/issues/6195
• I used DelegatingSpanData as a guide for this. I am not familiar enough with the OTEL codebase yet to know the full big picture, so if there's any code in there that doesn't make sense, that's why

[codecov[bot]]

Codecov Report

Attention: 35 lines in your changes are missing coverage. Please review.

Comparison is base (48d41d3) 91.05% compared to head (d75178a) 90.87%.

Files	Patch %	Lines
...lemetry/sdk/metrics/data/DelegatingMetricData.java	23.91%	26 Missing and 9 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6225      +/-   ##
============================================
- Coverage     91.05%   90.87%   -0.18%     
- Complexity     5687     5698      +11     
============================================
  Files           621      623       +2     
  Lines         16642    16699      +57     
  Branches       1703     1711       +8     
============================================
+ Hits          15153    15175      +22     
- Misses          998     1024      +26     
- Partials        491      500       +9     

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[jack-berg]

@ThomasJazz I'm going to close this because there hasn't been any activity in several months. Feel free to re-open if you pick this up again.