Add toBuilder() at PrometheusHttpServer

[asafm]

Per the conventions we have in the code, I've added the ability to create create a PrometheusHttpServerBuilder out of an existing PrometheusHttpServer.
This is particularly useful if you use AutoConfiguredOpenTelemetrySdkBuilder.addMetricReaderCustomizer(). In that method, you are given a MetricReader instance. In the case of PrometheusHttpServer you need to cast it to PrometheusHttpServer and then you can use the new toBuilder() to modify it.

This is a the completion step for (1) #6179 and (2) #6231

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.11%. Comparing base (98eded9) to head (b90e906).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6333      +/-   ##
============================================
+ Coverage     91.07%   91.11%   +0.03%     
- Complexity     5751     5760       +9     
============================================
  Files           626      627       +1     
  Lines         16782    16800      +18     
  Branches       1718     1718              
============================================
+ Hits          15285    15308      +23     
+ Misses         1003      998       -5     
  Partials        494      494              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[asafm]

@jack-berg Can you review?