Do not remove client span if incoming request has "uber-trace-id" header

[rafis]

Closes #340

[Copilot]

Pull Request Overview

This PR ensures that the client span is preserved when an incoming request contains the "uber-trace-id" header by modifying how headers are merged.

• Replace addAll with a loop that adds headers only if absent using the first value in each header.
• Change header insertion in the builder to use putIfAbsent, preventing overwriting of existing headers.

Comments suppressed due to low confidence (1)

instrument-starters/opentracing-spring-cloud-gateway-starter/src/main/java/io/opentracing/contrib/spring/cloud/gateway/TraceRequestHttpHeadersFilter.java:91

• Using putIfAbsent here prevents overwriting of existing headers, but may also bypass merging where multiple values are expected. Confirm that this logic is consistent with how header values should be combined during propagation.

headersWithInput.putIfAbsent(key, value);

[Copilot]

Only the first value of each header is preserved here, which may drop additional header values that might be required for some cases. Consider a solution that combines all values if preserving the full header list is necessary.

Suggested change

	headersWithInput.addIfAbsent(entry.getKey(), entry.getValue().get(0));
	headersWithInput.addAll(entry.getKey(), entry.getValue());