code: public synchronized static Tracer getTracer(String tracerName) {
          try {
              ExtensionClass<Tracer> ext = ExtensionLoaderFactory.getExtensionLoader(Tracer.class)
                  .getExtensionClass(tracerName);
              if (ext == null) {
                  throw ExceptionUtils.buildRuntime("tracer.name", tracerName);
              }
              return ext.getExtInstance();
          } catch (SofaRpcRuntimeException e) {
              throw e;
          } catch (Throwable e) {
              throw new SofaRpcRuntimeException(e.getMessage(), e);
          }
      }
docs: 初始化Tracer实例
  
  @return Tracer