Move hyper prometheus example into something runnable

[dmathieu]

This change moves the hyper-prometheus example out of the opentelemetry-prometheus, and into the global examples folder.

This is the only example located in a subpackage, making it harder to find.
That example also wasn't executable, making it harder to experiment on. By making the folder a part of the workspace, we can now run cargo run on it and see the behavior.

I've also changed the behavior a little bit to return the metrics only on the /metrics path, which is the usually expected behavior from a Prometheus exporter.

[codecov]

Codecov Report

Merging #562 (b3c3d12) into main (1235682) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #562     +/-   ##
=======================================
- Coverage   54.5%   54.5%   -0.1%     
=======================================
  Files         97      97             
  Lines       8534    8534             
=======================================
- Hits        4659    4658      -1     
- Misses      3875    3876      +1     

Impacted Files	Coverage Δ
opentelemetry/src/context.rs	92.1% <0.0%> (-0.8%)	⬇️

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[TommyCpp]

Good catch 👍 LGTM!