change attribute key name from "rule_name" to "sampling_rule_name"

# Issue
When a sampling rule is recorded on the `Segment`, the current attribute key is [rule_name](https://github.com/aws/aws-xray-sdk-java/blob/4034a834f0c580c9f661d096028f9bc654843aff/aws-xray-recorder-sdk-core/src/main/java/com/amazonaws/xray/entities/SegmentImpl.java#L133). The correct key name should be "sampling_rule_name"

# Reference
AWS X-Ray SDK for .NET/Core : [sampling_rule_name](https://github.com/aws/aws-xray-sdk-dotnet/blob/f41ad03fbec63b63a246bf76e4748c5fe346cadf/sdk/src/Core/AWSXRayRecorderImpl.cs#L372)