Add an option to dynamically add custom span attribute

[tigerwill90]

This PR introduces a new option that allows customization of span attributes based on the incoming request.

mw := New("foobar", WithTracerProvider(provider), WithSpanAttributes(func(r *http.Request) []attribute.KeyValue {
    attrs := make([]attribute.KeyValue, 1)
    attrs[0] = attribute.String("http.target", r.URL.String())
    return attrs
}))

This enhancement provides the ability to include non-standard attributes, such as http.target, in the span information. This can be particularly useful for capturing additional contextual information specific to the request.

[codecov]

Codecov Report

Merging #1 (f45b85a) into master (5a40de2) will increase coverage by 2.63%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   67.77%   70.40%   +2.63%     
==========================================
  Files           3        3              
  Lines          90       98       +8     
==========================================
+ Hits           61       69       +8     
  Misses         25       25              
  Partials        4        4              

Flag	Coverage Δ
coverage.txt	70.40% <100.00%> (+2.63%)	⬆️

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Impacted Files	Coverage Δ
foxtrace.go	75.86% <100.00%> (+1.78%)	⬆️
option.go	63.88% <100.00%> (+4.51%)	⬆️