Change the time used for X-Ray exception events to unix epoch time

- timestamp.String() generates a string like "2023-10-10 01:58:24.675761 +0000 UTC", which is not a standardized format
- Under the hood, it is just calling AsTime().String() https://github.com/open-telemetry/opentelemetry-collector/blob/pdata/v0.66.0/pdata/pcommon/timestamp.go#L36
- Time.String() has the warning "The returned string is meant for debugging; for a stable serialized representation, use t.MarshalText, t.MarshalBinary, or t.Format with an explicit format string." https://pkg.go.dev/time#Time.String

exporter/awsxrayexporter/internal/translator/cause.go

@@ -91,8 +91,8 @@ func makeCause(span ptrace.Span, attributes map[string]pcommon.Value, resource p
 				errorCode, ok1 := event.Attributes().Get(AwsIndividualHTTPErrorCodeAttr)
 				errorMessage, ok2 := event.Attributes().Get(AwsIndividualHTTPErrorMsgAttr)
 				if ok1 && ok2 {
-					timestamp := event.Timestamp().String()
-					strs := []string{errorCode.AsString(), timestamp, errorMessage.Str()}
+					eventEpochTime := event.Timestamp().AsTime().Unix()
+					strs := []string{errorCode.AsString(), strconv.FormatUint(uint64(eventEpochTime), 10), errorMessage.Str()}
 					message = strings.Join(strs, "@")
 					segmentID := newSegmentID()
 					exception := awsxray.Exception{

exporter/awsxrayexporter/internal/translator/cause_test.go

@@ -70,7 +70,7 @@ func TestMakeCauseAwsSdkSpan(t *testing.T) {
 	event1.SetName(AwsIndividualHTTPEventName)
 	event1.Attributes().PutStr(AwsIndividualHTTPErrorCodeAttr, "503")
 	event1.Attributes().PutStr(AwsIndividualHTTPErrorMsgAttr, "service is temporarily unavailable")
-	timestamp := pcommon.NewTimestampFromTime(time.Now())
+	timestamp := pcommon.NewTimestampFromTime(time.Unix(1696954760, 0))
 	event1.SetTimestamp(timestamp)
 
 	res := pcommon.NewResource()
@@ -88,7 +88,7 @@ func TestMakeCauseAwsSdkSpan(t *testing.T) {
 
 	messageParts := strings.SplitN(*exception.Message, "@", 3)
 	assert.Equal(t, "503", messageParts[0])
-	assert.Equal(t, timestamp.String(), messageParts[1])
+	assert.Equal(t, "1696954760", messageParts[1])
 	assert.Equal(t, "service is temporarily unavailable", messageParts[2])
 }