docs(readme): exporter should be metricExporter

[hongbo-miao]

Which problem is this PR solving?

Just a small fix in the code in the readme. Based on the context, it should be metricExporter instead of not existing exporter. 😃

Short description of the changes

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

Merging #3688 (56abc02) into main (25bb985) will not change coverage.
The diff coverage is n/a.

❗ Current head 56abc02 differs from pull request most recent head 321475b. Consider uploading reports for the commit 321475b to get more accurate results

Additional details and impacted files

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  Coverage   93.70%   93.70%           
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  Files         277      277           
  Lines        8445     8445           
  Branches     1753     1753           
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  Hits         7913     7913           
  Misses        532      532           

[pichlermarc]

Thanks for fixing this 🙂