TraceIDRatioBased doesn't work with xray.NewIDGenerator

[danilobuerger]

The first 4 bytes of an xray trace id are time based (https://docs.aws.amazon.com/xray/latest/devguide/xray-api-sendingdata.html#xray-api-traceids):

opentelemetry-go-contrib/propagators/aws/xray/idgenerator.go

Lines 57 to 59 in ace6b3c

	tid := trace.TraceID{}
	currentTime := getCurrentTimeHex()
	copy(tid[:4], currentTime)

When using TraceIDRatioBased, it compares the first 8 bytes to see if it should sample:

https://github.com/open-telemetry/opentelemetry-go/blob/569f7430726278f40e562f81e64d21156cc3edb9/sdk/trace/sampling.go#L84-L85

This assumes that the first 8 bytes are random, which they are not. So if the current time is above the upper bound it will never sample.