chore(otelgin): rename files for improved clarity and consistency

[flc1125]

Same as them, adjust the naming to be more semantic. Moreover, currently, gin is not only about trace but also includes metric.

• https://github.com/open-telemetry/opentelemetry-go-contrib/tree/8b5220b6f51f51db8d05656b3c8ebce4e25ffbc5/instrumentation/github.com/labstack/echo/otelecho
• https://github.com/open-telemetry/opentelemetry-go-contrib/tree/8b5220b6f51f51db8d05656b3c8ebce4e25ffbc5/instrumentation/github.com/gorilla/mux/otelmux

Additional note: This should be able to skip the CHANGELOG.

[MrAlias]

Not sure this is technically "the correct naming", but seems like an okay change.

[flc1125]

Not sure this is technically "the correct naming", but seems like an okay change.

It may not be the best name in terms of technology. More to unify it with other packages.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.5%. Comparing base (a3c993d) to head (8526574).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6786   +/-   ##
=====================================
  Coverage   75.5%   75.5%           
=====================================
  Files        206     206           
  Lines      19181   19181           
=====================================
+ Hits       14484   14487    +3     
+ Misses      4259    4257    -2     
+ Partials     438     437    -1     

Files with missing lines	Coverage Δ
...ntation/github.com/gin-gonic/gin/otelgin/config.go	100.0% <ø> (ø)
...umentation/github.com/gin-gonic/gin/otelgin/gin.go	92.4% <ø> (ø)

... and 1 file with indirect coverage changes