code: function(providers, method_definition) {
    if (!Array.isArray(providers)) {
      return null;
    }
    var interceptors = [];
    for (var i = 0; i < providers.length; i++) {
      var provider = providers[i];
      var interceptor = provider(method_definition);
      if (interceptor) {
        interceptors.push(interceptor);
      }
    }
    return interceptors;
  }
docs: Transforms a list of interceptor providers into interceptors.
  @param {InterceptorProvider[]} providers
  @param {grpc~MethodDefinition} method_definition
  @return {null|Interceptor[]}