refactor: move example from otelmux to make it a testable example

[noetarbouriech]

Resolves #7064

I simply moved the old example to have it shown on the pkg.go.dev page. It will require testing for metrics in the future.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: noetarbouriech / name: Noé Tarbouriech (c4885aa, 673a0af, 31503a1, 2aaa827)
• ✅ login: dmathieu / name: Damien Mathieu (94a8c4b)
• ✅ login: pellared / name: Robert Pająk (6bad0e8, 4345696, 08aeb97, 85ac8fa, 1afe639)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.6%. Comparing base (158bafc) to head (94a8c4b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7121     +/-   ##
=======================================
+ Coverage   81.5%   81.6%   +0.1%     
=======================================
  Files        206     205      -1     
  Lines      18270   18231     -39     
=======================================
- Hits       14891   14885      -6     
+ Misses      2959    2924     -35     
- Partials     420     422      +2     

see 2 files with indirect coverage changes

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[dmathieu]

Could you fix the conflicts?

[dmathieu]

Ping @noetarbouriech to update the PR so we can review/merge it?