You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: _kafka_action_header.rs
    path of file: ./repos/aws-sdk-rust/sdk/iot/src/types
    
    the code of the file until where you have to start completion: // Code generated by software.amazon.smithy.rust.codegen.smithy-rs. DO NOT EDIT.

/// <p>Specifies a Kafka header using key-value pairs when you create a Ruleâ€™s Kafka Action. You can use these headers to route data from IoT clients to downstream Kafka clusters without modifying your message payload.</p>
/// <p>For more information about Rule's Kafka action, see <a href="https://docs.aws.amazon.com/iot/latest/developerguide/apache-kafka-rule-action.html">Apache Kafka</a>.</p>

    
    pub fn build(self) -> ::std::result::Result<crate::types::KafkaActionHeader, ::aws_smithy_types::error::operation::BuildError> {
        ::std::result::Result::Ok(crate::types::KafkaActionHeader {
            key: self.key.ok_or_else(|| {
                ::aws_smithy_types::error::operation::BuildError::missing_field(
                    "key",
                    "key was not specified but it is required when building KafkaActionHeader",
                )
            })?,
            value: self.value.ok_or_else(|| {
                ::aws_smithy_types::error::operation::BuildError::missing_field(
                    "value",
                    "value was not specified but it is required when building KafkaActionHeader",
                )
            })?,
        })
    }