Fix benchmarking metrics data upload request issue

[ambushwork]

What does this PR do?

Fix metrics data upload due to the modification of request body media type,

Actually "Metrics" and "Span" require different media type as intake.

"Span" API requires plain text so several span json can be combined in the same request, while "Metrics" API requires Json format, otherwise it refuses the request with 400 Bad request response.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.96%. Comparing base (87cc448) to head (5da01c1).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2227      +/-   ##
===========================================
- Coverage    70.02%   69.96%   -0.06%     
===========================================
  Files          727      727              
  Lines        27036    27044       +8     
  Branches      4556     4557       +1     
===========================================
- Hits         18931    18920      -11     
- Misses        6828     6838      +10     
- Partials      1277     1286       +9     

see 38 files with indirect coverage changes