Use explicitly custom code snippet examples in otelecho

[eddyariki]

Addresses #8373

• Adds comment in echo_test.go to explicitly recommend avoiding using high-cardinality attributes
• Replaces custom metrics example in WithMetricAttributeFn and WithEchoMetricAttributeFn with safer examples.
• Showcases behavior with duplicate metrics as well.

Note:

• Most standard metrics for http.Request and echo context are already added as attributes in the middleware, so I went with a more custom metric for the sake of high-lighting potential use-cases.
• I checked other code snippets and only found echo_test.go's dangerous pattern in their tests. I believe there are no other dangerous patterns in the core packages and instrumentation libraries.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (4726bd3) to head (9fe7867).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8381   +/-   ##
=====================================
  Coverage   82.3%   82.3%           
=====================================
  Files        182     182           
  Lines      13764   13764           
=====================================
  Hits       11329   11329           
  Misses      2030    2030           
  Partials     405     405           

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[flc1125]

There's nothing wrong with it overall.