Retry metric middleware probably shouldn't return error if no retry metadata

[jasdel]

The retry metric middleware will return an error if it cannot find the retry metadata in the middleware context. This middleware should most likely just forward on the request instead of errorring out.

https://github.com/aws/aws-sdk-go-v2/blob/223893cb4d9d2da39d7f82d87a8614f0f8c95bbe/aws/retry/middleware.go#L132-L135

[jasdel]

created in v2 SDK repo.