io/read_metadata: Fix dtype for the index column

[joverlee521]

Description of proposed changes

Ensure that the dtype for the index column is always a string so that it can be properly matched with ids even if they are numeric values.

Related issue(s)

Resolves #925

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.06%. Comparing base (325b37b) to head (489fa92).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1746   +/-   ##
=======================================
  Coverage   73.06%   73.06%           
=======================================
  Files          79       79           
  Lines        8335     8335           
  Branches     1700     1700           
=======================================
  Hits         6090     6090           
  Misses       1958     1958           
  Partials      287      287           

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[victorlin]

Oh no, this bug has been around since 8161060, 1 year!