Xray subsegment

[rexnp]

note: this is a follow up PR to #2. The changes made from previous are:

1. set child span name to operation name instead of using the parent span's name
2. updated doc

Commit Message: xray tracer: sets the xray segment with type "subsegment" for child spans.
xray doc: https://docs.aws.amazon.com/xray/latest/devguide/xray-api-segmentdocuments.html#api-segmentdocuments-subsegments
Additional Description:
Risk Level: low
Testing: unit test, manual setup verifying new xray behavior
Docs Changes: added entry in version history under bug fixes
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue] aws/aws-app-mesh-roadmap#354
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

below is a sample XRay segment traced by the proxy that includes a subsegment calling an upstream. without this change the subsegment would be emitted as a separate segment, which would mess up the XRay service map.

{
            "Id": "8eecc8a82d35c902",
            "Document": {
                "id": "8eecc8a82d35c902",
                "name": "howto-ecs-basics/howto-ecs-basics-front-node",
                "start_time": 1644451809.443302,
                "trace_id": "1-620457e1-47c3587c6ede1c08098a7273",
                "end_time": 1644451809.4465587,
                "parent_id": "257d8a179102651d",
                "http": {
                    "request": {
                        "url": "http://color.howto-ecs-basics.mesh.local:8080/",
                        "method": "GET",
                        "user_agent": "Go-http-client/1.1",
                        "client_ip": "10.0.77.8",
                        "x_forwarded_for": false
                    },
                    "response": {
                        "status": 200,
                        "content_length": 5
                    }
                },
                "aws": {
                    "app_mesh": {
                        "mesh_name": "howto-ecs-basics",
                        "virtual_node_name": "howto-ecs-basics-front-node"
                    }
                },
                "annotations": {
                    "response_flags": "-",
                    "component": "proxy",
                    "upstream_cluster": "cds_egress_howto-ecs-basics_howto-ecs-basics-color-node_http_8080",
                    "request_size": "0",
                    "downstream_cluster": "-",
                    "node_id": "mesh/howto-ecs-basics/virtualNode/howto-ecs-basics-front-node",
                    "direction": "egress"
                },
                "origin": "AWS::AppMesh::Proxy",
                "subsegments": [
                    {
                        "id": "e6badfd202e93ce9",
                        "name": "router cds_egress_howto-ecs-basics_howto-ecs-basics-color-node_http_8080 egress",
                        "start_time": 1644451809.4434495,
                        "end_time": 1644451809.446304,
                        "http": {
                            "request": {},
                            "response": {
                                "status": 200
                            }
                        },
                        "aws": {},
                        "annotations": {
                            "response_flags": "-",
                            "component": "proxy",
                            "upstream_cluster": "cds_egress_howto-ecs-basics_howto-ecs-basics-color-node_http_8080",
                            "upstream_address": "10.0.66.192:8080",
                            "direction": "egress"
                        }
                    }
                ]
            }
        }

[abaptiste]

LGTM. Made one small suggestion.