Updated Documentation For Change in Behaviour of Reading X-Ray Tracin…

…g Info From Lambda (#27)

Co-authored-by: Tyler Benson <tylerbenson@gmail.com>
Co-authored-by: Christian Neumüller <christian+github@neumueller.me>
Co-authored-by: Josh Suereth <joshuasuereth@google.com>

CHANGELOG.md

@@ -7,6 +7,8 @@ release.
 
 ## Unreleased
 
+- Updated AWS Java Lambda guidance - using system properties.
+
 ### Semantic Conventions
 
 - Clarify the scope of the HTTP client span.

specification/trace/semantic_conventions/instrumentation/aws-lambda.md

@@ -66,6 +66,8 @@ contain an incomplete trace context which indicates X-Ray isn’t enabled. The e
 `Context` will be valid and sampled only if AWS X-Ray has been enabled for the Lambda function. A user can
 disable AWS X-Ray for the function if the X-Ray Span Link is not desired.
 
+**Note**: When instrumenting a Java AWS Lambda, instrumentation SHOULD first try to parse an OpenTelemetry `Context` out of the system property `com.amazonaws.xray.traceHeader` using the [AWS X-Ray Propagator](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md) before checking and attempting to parse the environment variable above.
+
 [Span Link]: https://opentelemetry.io/docs/concepts/signals/traces/#span-links
 
 ## API Gateway