[thomascorthals]

Closes #669.

I've modelled this on Solr 10 which overhauled the query parameters for Metrics queries. It also did away with the JSON response format and introduced OpenMetrics 1.0 alongside the now default Prometheus format. To parse this response into an object, I've suggested butschster/prometheus-parser. Without this library, it's still possible to get the raw output as shown in the examples.

For older Solr versions, you can explicitly set WT_JSON on the query and get the ouput as an array, but not as Result object. This is also shown in the examples.

As a side quest, I've added Utility::stringOrArrayToArray() because I found myself repeating the same code for methods that take 'one, two' or ['one', 'two'] as equivalent ways of passing the same options.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.40%. Comparing base (1acb7db) to head (fdfe81d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
- Coverage   98.42%   98.40%   -0.02%     
==========================================
  Files         403      406       +3     
  Lines       10659    10676      +17     
==========================================
+ Hits        10491    10506      +15     
- Misses        168      170       +2     

Flag	Coverage Δ
unittests	98.40% <100.00%> (-0.02%)	⬇️

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