Updated AWS lambda docs to use ARN for layer installation (elastic#2467)

Author: AlexanderWert

docs/setup-aws-lambda.asciidoc

@@ -9,16 +9,6 @@ and there are various ways you can tweak it to fit your needs.
 NOTE: In order to get the full AWS Lambda tracing capabilities, use with APM Server 7.16 or higher. Using
 older versions provides most value, but some relevant metadata fields will not be indexed.
 
-[float]
-[[aws-lambda-installation]]
-==== Installation
-
-Setting up your Lambda function is a three-step process.
-
-1. Check <<aws-lambda-runtimes>>
-2. <<aws-lambda-extension>>
-3. <<aws-lambda-instrumenting>>
-
 [float]
 [[aws-lambda-runtimes]]
 ==== Supported runtimes
@@ -49,41 +39,42 @@ explicitly.
 
 |===
 
+[float]
+[[aws-lambda-installation]]
+==== Installation
+
+Setting up APM for your Java Lambda function is a two-step process. Make sure to follow _both_ of the following steps:
+
+1. <<aws-lambda-extension>>
+2. <<aws-lambda-instrumenting>>
+
 [float]
 [[aws-lambda-extension]]
 ==== Install the Elastic APM Lambda extension
 
-Elastic uses a Lambda extension to forward data to APM Server in a way that does not interfere with the
-execution of your Lambda function.
-
-See the https://www.elastic.co/guide/en/apm/guide/current/aws-lambda-extension.html[installation documentation] to get started.
+Elastic uses a Lambda extension to forward data to an APM Server in a way that does not interfere with the execution of your Lambda function.
 
-NOTE: Not all environment variables specified within https://www.elastic.co/guide/en/apm/guide/current/aws-lambda-extension.html#aws-lambda-variables[The Necessary Variables]
-section of the extension documentation need to be configured. The Java agent utilizes a https://docs.aws.amazon.com/lambda/latest/dg/runtimes-modify.html[wrapper script]
-to set some of these up, so follow the next section to see which are required.
+Follow the https://www.elastic.co/guide/en/apm/guide/current/aws-lambda-extension.html[installation documentation for the extension] to setup the Elastic APM Lambda extension for your Lambda function.
 
 [float]
 [[aws-lambda-instrumenting]]
-==== Installing the Elastic APM Java Agent layer
-
-The Java Agent is installed as an AWS Layer as well. It is distributed as a ZIP archive containing two files:
-
-- The Java Agent jar file
-- A wrapper script that sets up the runtime to enable automatic attachment of the agent
-
-Follow these steps to ensure proper Java Agent attachment to your Lambda function:
-
-1. Download the `elastic-apm-java-aws-lambda-layer-<VERSION>.zip` archive from our
-https://github.com/elastic/apm-agent-java/releases[GitHub Releases page].
-2. Publish the downloaded ZIP archive as an https://docs.aws.amazon.com/lambda/latest/dg/invocation-layers.html?icmpid=docs_lambda_help[AWS Lambda Layer].
-3. Configure your function to use the Java Agent layer.
-4. Within your Function configuration, configure the following environment variables:
-a. `AWS_LAMBDA_EXEC_WRAPPER` (required): set this variable's value to `/opt/elastic-apm-handler`, so that Lambda starts the runtime with this script.
-b. `ELASTIC_APM_LAMBDA_APM_SERVER` (required): this will tell the Elastic APM Extension where to send data to.
-c. <<config-secret-token, `ELASTIC_APM_SECRET_TOKEN`>> or <<config-api-key, `ELASTIC_APM_API_KEY`>> (required): one of these needs to be set
-as the authentication method that the extension uses when sending data to the URL configured via `ELASTIC_APM_LAMBDA_APM_SERVER`
-d. <<config-application-packages, `ELASTIC_APM_APPLICATION_PACKAGES`>> (recommended): setting this may improve cold start times.
-5. Fine-tune the Java agent with any of the available <<configuration, configuration options>>.
+==== Install the Elastic APM Java Agent Layer
+
+The Java Agent is installed as an AWS Layer. 
+
+1. Pick the right ARN from the https://github.com/elastic/apm-agent-java/releases[ARN table for the APM Java Agent Lambda Layer]. Note: The APM Java Agent Layer needs to be in the _same region_ as your Lambda function.
+2. Navigate to your function in the AWS Console
+3. Scroll to the Layers section and click the _Add a layer_ button
+4. Choose the _Specify an ARN_ radio button
+5. Enter the Version ARN of the APM Java Agent Layer in the _Specify an ARN_ text input
+6. Click the _Add_ button
+
+Once the APM Java Agent Layer is in place, you'll need to configure some environment variables _in addition_ to the environment variables you already configured for the APM Lambda extension. 
+Configure the following environment variables for the APM Java Agent Layer:
+
+1. (required) `AWS_LAMBDA_EXEC_WRAPPER`: set this variable's value to `/opt/elastic-apm-handler`, so that Lambda starts the runtime with an attached APM Java agent.
+2. (recommended) <<config-application-packages, `ELASTIC_APM_APPLICATION_PACKAGES`>>: setting this may improve cold start times.
+3. (optional) Fine-tune the Java agent with any of the available <<configuration, configuration options>>.
 
 That's it, if you've done all of the above you are set to go!
 Your Lambda function invocations should be traced from now on.