[chore][testonly][exporter] Use new{Logs,Metrics,Traces}Request instead of &{logs,metrics,traces}Request{}

[sfc-gh-sili]

Description

As part of the effort to cache request size to improve merge splitting performance
(See #12137 and #12136), we want to make sure requests are created from create function instead of getting initialized via struct directly.

• newLogsRequest instead of &logsRequest{}
• newMetrcsRequest instead of &metricsRequest{}
• newTracesRequest instead of &tracesRequest{}

Link to tracking issue

#12137

Testing

Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.73%. Comparing base (b09a65b) to head (4230ac3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12152   +/-   ##
=======================================
  Coverage   91.73%   91.73%           
=======================================
  Files         463      463           
  Lines       24819    24819           
=======================================
  Hits        22767    22767           
  Misses       1670     1670           
  Partials      382      382           

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