Change tests to use the new internal metrics structs

[bogdandrutu]

Updates #1657

[codecov]

Codecov Report

Merging #1695 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1695      +/-   ##
==========================================
- Coverage   92.32%   92.31%   -0.02%     
==========================================
  Files         265      265              
  Lines       19811    19811              
==========================================
- Hits        18290    18288       -2     
- Misses       1091     1092       +1     
- Partials      430      431       +1     

Impacted Files	Coverage Δ
receiver/otlpreceiver/otlp.go	91.66% <0.00%> (-2.78%)	⬇️

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