在单元测试用例中模拟跨度-Zipkin / brave

Question

我将Brave用作Spring Slueth（Spring Cloud-Edgware.RELEASE）的一部分。我编写了一个过滤器，该过滤器可拦截每个请求并在Zipkin中创建一个Span（标记/名称）。例如：

//Imports
import brave.Span;
import brave.Tracer;

@Override
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {

    Span someSpan = tracer.nextSpan();
    someSpan.kind(Span.Kind.SERVER);

    someSpan.name("my-test-span");
    someSpan.tag("Env", "Dev");

    //Do some Filter specific operation ......

    someSpan.finish();

}

现在，我想编写测试用例，但是我不确定如何在Servlet过滤器中模拟Span或tracer？

POM片段：              

        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-sleuth</artifactId>
            <version>2.0.3.BUILD-SNAPSHOT</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>

        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>Edgware.RELEASE</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>

    </dependencies>
</dependencyManagement>

<dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-zipkin</artifactId>
</dependency>