feat: Support mixed categories for semconv opt-in

[prestonvasquez]

Resolves #7246

Introducing support for category-specific values in the OTEL_SEMCONV_STABILITY_OPT_IN environment variable for otelhttp and otelmongo. For example, multiple categories should be supported without error:

export OTEL_SEMCONV_STABILITY_OPT_IN=http,database,messaging/dup

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.1%. Comparing base (046b3b6) to head (4963776).
Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7274   +/-   ##
=====================================
  Coverage   81.0%   81.1%           
=====================================
  Files        204     204           
  Lines      18095   18188   +93     
=====================================
+ Hits       14672   14762   +90     
- Misses      3003    3005    +2     
- Partials     420     421    +1     

Files with missing lines	Coverage Δ
...lei/go-restful/otelrestful/internal/semconv/env.go	94.9% <100.0%> (+0.1%)	⬆️
....com/gin-gonic/gin/otelgin/internal/semconv/env.go	94.9% <100.0%> (+0.1%)	⬆️
...ub.com/gorilla/mux/otelmux/internal/semconv/env.go	93.0% <100.0%> (+0.2%)	⬆️
.../mongo/otelmongo/internal/semconv/event_monitor.go	100.0% <100.0%> (ø)
...tp/httptrace/otelhttptrace/internal/semconv/env.go	90.5% <100.0%> (+0.3%)	⬆️
...entation/net/http/otelhttp/internal/semconv/env.go	90.5% <100.0%> (+0.3%)	⬆️

... and 3 files with indirect coverage changes

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[dmathieu]

Maybe a nit, but using Contains means writing down http/dupdatabase works. So does http/dup;database.
Should we ensure the string is properly separated by commas? (obviously, this has a performance impact, so I'm also fine saying it doesn't matter).

[flc1125]

Actually, I have this issue too. But since it only needs to be executed once, I think the overhead is worth it.

[flc1125]

A simple and very strange way: strings.Contains(","env+",", ",http/dup,")

Not for reference or advice.