Add echo-opentelemetry to the README.md

[aldas]

Add https://github.com/labstack/echo-opentelemetry to the middleware list in README.md

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.01%. Comparing base (19364e2) to head (ec8d086).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2908   +/-   ##
=======================================
  Coverage   93.01%   93.01%           
=======================================
  Files          43       43           
  Lines        4496     4497    +1     
=======================================
+ Hits         4182     4183    +1     
  Misses        196      196           
  Partials      118      118           

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