Minor improvements to Prometheus Exporter (ASP.NET Core)

[reyang]

Changes

• Changed the default cache duration from 10 seconds to 0.3 second (which is a good enough default for machine, and yet provide good interactive feedback to human).
• Improved the docs.

Given this is a fairly small change on the not-yet-released package, I didn't include a changelog entry.

[codecov]

Codecov Report

Merging #3522 (bb06572) into main (27658f0) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3522      +/-   ##
==========================================
- Coverage   86.79%   86.73%   -0.07%     
==========================================
  Files         275      275              
  Lines        9958     9958              
==========================================
- Hits         8643     8637       -6     
- Misses       1315     1321       +6     

Impacted Files	Coverage Δ
...Prometheus.AspNetCore/PrometheusExporterOptions.cs	50.00% <100.00%> (ø)
...tation/OpenTelemetryProtocolExporterEventSource.cs	75.00% <0.00%> (-10.00%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
src/OpenTelemetry/BatchExportProcessor.cs	82.24% <0.00%> (-1.87%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.87% <0.00%> (-0.79%)	⬇️