Adding interceptor support so trace propagation can be plugged

[salaboy]

Supporting pluggable OK Http interceptor to for example support trace propagation

For example, to propagate X-Trace-Id from an incoming request

In a Spring Boot / Servlet filter context

  // Shared ThreadLocal — set this from your servlet filter or interceptor                                                                                    
  public static final ThreadLocal<String> TRACE_ID = new ThreadLocal<>();                                                                                       
                                                                                                                                                                
  // Build the client once at startup                                                                                                                           
  OkHttpClient httpClient = OkHttpClient.builder()                                                                                                              
      .backend(backend)                                                                                                                                         
      .addInterceptor(chain -> {
          String traceId = TRACE_ID.get();                                                                                                                      
          okhttp3.Request request = traceId != null                                                                                                           
              ? chain.request().newBuilder().addHeader("X-Trace-Id", traceId).build()
              : chain.request();                                                                                                                                
          return chain.proceed(request);
      })                                                                                                                                                        
      .build();                                                                                                                                               

  AnthropicClient client = AnthropicOkHttpClient.builder()                                                                                                      
      .apiKey("...")
      .httpClient(httpClient)                                                                                                                                   
      .build();                                                                                                                                               

  // In your Spring HandlerInterceptor or servlet Filter:                                                                                                       
  TRACE_ID.set(request.getHeader("X-Trace-Id"));
  try {                                                                                                                                                         
      // handle request — the Anthropic client will forward the header                                                                                          
  } finally {
      TRACE_ID.remove(); // prevent leaks across thread reuse                                                                                                   
  }      

With OpenTelemetry / Brave (if already using a tracing framework)

If you're already using OTel or Brave, you can use their context propagators directly instead of ThreadLocal:

  .addInterceptor(chain -> {                                                                                                                                    
      // OTel example                                                                                                                                         
      Span span = Span.current();
      String traceId = span.getSpanContext().getTraceId();                                                                                                      
      return chain.proceed(
          chain.request().newBuilder().addHeader("X-Trace-Id", traceId).build()                                                                                 
      );                                                                                                                                                      
  })