Allow custom counters implementing CounterInterface to be exported

[erdemkose]

PrometheusExporter::doExport() only allows instances of OpenTelemetry\SDK\Metrics\Counter class to be exported. This makes it impossible to export counters extending the Counter class or just implementing CounterInterface.

Allowing any counters implementing CounterInterface to be exported would make things easier.

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• ✅ login: erdemkose / name: Erdem Köse (cb5f58f)

[bobstrecansky]

@erdemkose - just a CLA sign and a style check fix and I think we'll be off to the races with this PR 👍🏼

[codecov]

Codecov Report

Merging #676 (cb5f58f) into main (85282b5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #676   +/-   ##
=========================================
  Coverage     86.07%   86.07%           
  Complexity     1151     1151           
=========================================
  Files           128      128           
  Lines          2794     2794           
=========================================
  Hits           2405     2405           
  Misses          389      389           

Flag	Coverage Δ
7.4	86.03% <100.00%> (ø)
8.0	86.07% <100.00%> (ø)
8.1	86.07% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Contrib/Prometheus/PrometheusExporter.php	93.75% <100.00%> (ø)

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[erdemkose]

@erdemkose - just a CLA sign and a style check fix and I think we'll be off to the races with this PR 👍🏼

@bobstrecansky done and done 👍