Add `Metadata` to a config layer through operation runtime plugin (#2830)

## Motivation and Context
Adds `Metadata` to a config layer through `OperationRuntimePlugin`.

## Description
We have had a customer's use case where a service name and a operation
name are obtained from
[Metadata](https://github.com/awslabs/smithy-rs/blob/ddba46086a9754c01f3b11d7521c49d4489de84b/rust-runtime/aws-smithy-http/src/operation.rs#L17-L22

).
The end goal was to use their names as part of metrics collection.
Previously, it was done using `map_operation` on a
`CustomizableOperation`, e.g.
```
client
    .some_operation()
    .customize()
    .await?
    .map_operation(|operation| {
        operation.try_clone().map(|operation| {
            let (_, parts) = operation.into_request_response();
            parts.metadata.map(|metadata| {
                let service_name = metadata.service().to_string().to_uppercase();
                let operation_name = metadata.name().to_string();
                /*
                 * do something with `service_name` and `operation_name`
                 */
            })
        });

        Ok(operation)
    })?
    .send()
    .await;
```
The orchestrator no longer supports `map_operation` on
`CustomizableOperation`. We therefore add `Metadata` to a config layer
through `OperationRuntimePlugin`. See the added integration test for how
`Metadata` is retrieved from within an interceptor.

## Testing
Added an integration-test to verify `Metadata` is properly set.

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Co-authored-by: ysaito1001 <awsaito@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>