[MINOR] set content-type to application/json for metadata endpoint

Christopher Schinnerlrequested to merge
chris/metadata-application-json into master

MERGE REQUEST

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Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

  • All new methods or updated methods have clear docstrings
  • Testing added or updated for new methods
  • Any new packages are added to Makefile and .gitlab-ci.yml
  • API documentation updated for API updates
  • Module README.md updated for changes to workflow
  • Changelog File Created

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