[Metrics SDK] Small fix in variable names in Sync Instrument unit tests

[lalitb]

Changes

Found while doing some performance tests on metrics.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1987 (7f5b8e0) into main (e47dec5) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 7f5b8e0 differs from pull request most recent head a199036. Consider uploading reports for the commit a199036 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1987      +/-   ##
==========================================
+ Coverage   87.11%   87.13%   +0.03%     
==========================================
  Files         166      166              
  Lines        4597     4597              
==========================================
+ Hits         4004     4005       +1     
+ Misses        593      592       -1     

Impacted Files	Coverage Δ
sdk/src/trace/batch_span_processor.cc	91.48% <0.00%> (+0.78%)	⬆️

[marcalff]

LGTM