Test to validate identical instruments are merged

[cijothomas]

Continuing from #1382, this adds one more test, validating a different aspect - instruments with matching identity must be merged to produce same metric stream.

I may add few more of similar tests (testing diff. aspects like view/callbacks etc.), and once all is done, we can identify common parts of these tests and move common code to helpers as opposed to duplicating a lot of code in each test. Also it'll be possible to test multiple things in single test.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7848755) 57.2% compared to head (86346e0) 57.2%.

Files	Patch %	Lines
opentelemetry-sdk/src/metrics/mod.rs	97.8%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1385   +/-   ##
=====================================
  Coverage   57.2%   57.2%           
=====================================
  Files        146     146           
  Lines      18079   18126   +47     
=====================================
+ Hits       10343   10376   +33     
- Misses      7736    7750   +14     

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