otelhttp: deprecate the WithRouteTag option

[dmathieu]

This is part of #8107

The WithRouteTag option doesn't really have any needs anymore.
For spans, we already add the route name automatically. For metrics, folks can use the WithMetricAttributesFn option.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.0%. Comparing base (8d65e49) to head (968cc14).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8117   +/-   ##
=====================================
  Coverage   80.9%   81.0%           
=====================================
  Files        194     194           
  Lines      13073   13073           
=====================================
+ Hits       10588   10592    +4     
+ Misses      2107    2105    -2     
+ Partials     378     376    -2     

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/handler.go	92.7% <ø> (ø)

... and 1 file with indirect coverage changes

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[pellared]

This should be

Suggested change

	// To annotate metrics, use the `WithMetricAttributesFn` option.
	// To annotate metrics, use the [WithMetricAttributesFn] option.