Optionally disable timestamps in the Prometheus exporters

[hannahhaering]

Implements #5485

Changes

• Added the possibility to disable timestamps in the Prometheus HttpListener and Prometheus AspNetCore exporter
• In both cases the timestamp can be disabled via the corresponding options

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.70%. Comparing base (84c0f42) to head (d1d4576).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6600      +/-   ##
==========================================
- Coverage   86.76%   86.70%   -0.07%     
==========================================
  Files         258      258              
  Lines       11943    11955      +12     
==========================================
+ Hits        10362    10365       +3     
- Misses       1581     1590       +9     

Flag	Coverage Δ
unittests-Project-Experimental	86.64% <100.00%> (-0.08%)	⬇️
unittests-Project-Stable	86.59% <100.00%> (-0.14%)	⬇️
unittests-UnstableCoreLibraries-Experimental	86.03% <100.00%> (+0.15%)	⬆️

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

Files with missing lines	Coverage Δ
...ometheus.AspNetCore/PrometheusAspNetCoreOptions.cs	62.50% <100.00%> (+12.50%)	⬆️
...tpListener/Internal/PrometheusCollectionManager.cs	86.20% <100.00%> (+0.09%)	⬆️
...etheus.HttpListener/Internal/PrometheusExporter.cs	100.00% <100.00%> (ø)
...HttpListener/Internal/PrometheusExporterOptions.cs	100.00% <100.00%> (ø)
...s.HttpListener/Internal/PrometheusSerializerExt.cs	100.00% <100.00%> (ø)
...theusHttpListenerMeterProviderBuilderExtensions.cs	81.81% <100.00%> (+0.56%)	⬆️
...heus.HttpListener/PrometheusHttpListenerOptions.cs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[Kielek]

@hannahhaering, thanks!

[github-actions]

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