Add middleware option to supply StartSpanOptions

[vpranckaitis]

This commit adds a middleware option that supplies StartSpanOptions for StartSpan() call.

Concrete use case where this is useful: adding additional tags to the span through opentracing.Tag{} option, so that those tags could be used for deciding whether to sample this span.

[vpranckaitis]

@lucacome drawing your attention to this since you made most of the recent contributions. Does this change make sense?

[Copilot]

Pull Request Overview

This PR adds a middleware option to supply custom StartSpanOptions for tracing, enabling users to append additional tags (e.g. via opentracing.Tag) to spans for dynamic sampling decisions.

• Introduces the MWStartSpanOptions middleware option and its corresponding helper function.
• Adds a new test (TestStartSpanOptionsOption) to verify that the custom StartSpanOptions are correctly applied.
• Updates the MiddlewareFunc and supporting collectStartSpanOptions function in server.go to integrate the new option.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
nethttp/server_test.go	New test case added to validate the behavior of the StartSpanOptions.
nethttp/server.go	New MWStartSpanOptions option added and integrated into the middleware.

Comments suppressed due to low confidence (2)

nethttp/server_test.go:250

• [nitpick] Using the string literal "" to represent a missing tag in the test may lead to confusion. Consider using an empty string or a different sentinel value to clearly indicate the absence of the tag.

if !ok {

nethttp/server.go:183

• [nitpick] The default RPCServerOption is prepended before user-supplied options, which may limit users' ability to override its behavior. Consider documenting the ordering or allowing user options to take precedence if intended.

startSpanOptions = append(startSpanOptions, mwStartSpanOptions...)

[vpranckaitis]

@lucacome any comments? This one from copilot seem like a false-positive:

[nitpick] Using the string literal "" to represent a missing tag in the test may lead to confusion. Consider using an empty string or a different sentinel value to clearly indicate the absence of the tag.

The test already uses "<nil>" sentinel value.

[vpranckaitis]

@lucacome in case you do not have time, maybe there's someone else who could do the review of this PR?