otelhttp: Refactor Option interface

[pellared]

Why

Adhere to https://github.com/open-telemetry/opentelemetry-go/blob/main/CONTRIBUTING.md#configuration

What

Unexport Option.Apply.

[codecov]

Codecov Report

Merging #750 (811ed45) into main (01fdab7) will increase coverage by 0.6%.
The diff coverage is 80.5%.

@@           Coverage Diff           @@
##            main    #750     +/-   ##
=======================================
+ Coverage   78.4%   79.0%   +0.6%     
=======================================
  Files         62      62             
  Lines       2737    2726     -11     
=======================================
+ Hits        2146    2154      +8     
+ Misses       456     440     -16     
+ Partials     135     132      -3     

Impacted Files	Coverage Δ
detectors/gcp/gce.go	7.3% <0.0%> (ø)
detectors/gcp/gke.go	0.0% <0.0%> (ø)
exporters/metric/dogstatsd/internal/statsd/conn.go	59.2% <ø> (ø)
...github.com/aws/aws-sdk-go-v2/otelaws/attributes.go	100.0% <ø> (ø)
instrumentation/host/host.go	73.1% <ø> (ø)
...umentation/net/http/httptrace/otelhttptrace/api.go	100.0% <ø> (ø)
...on/net/http/httptrace/otelhttptrace/clienttrace.go	76.2% <ø> (ø)
instrumentation/net/http/otelhttp/labeler.go	84.6% <ø> (ø)
detectors/aws/eks/detector.go	32.7% <33.3%> (+3.6%)	⬆️
...ion/google.golang.org/grpc/otelgrpc/interceptor.go	87.0% <87.5%> (ø)
... and 21 more

[pellared]

@MadVikingGod Based on #746 (comment) I think you will like this PR 😉

[MrAlias]

I would like to see our configuration style guide updated before this is merged, but in general I'm in favor of this change.

[pellared]

@Aneurysm9 @MrAlias Can you take another look as the Option design is already documented?