fix [core] trace level shows the first kb of the payload before being decoded

[sebsto]

Add a log statement in the Lambda loop, before calling the user's handler to show the raw payload before any attempt to decode it.

This was available in Runtime v1 and is now ported to v2.
This fixes #404

Motivation:

This is useful when handling custom event and there is a Decoding error. It allows to see the exact payload received by the handler before any attempt to decode it.

Modifications:

Add a log.trace statement with metatadata. Metadata are computed only when the log level is trace or below.

Result:

2025-07-21T08:58:33+0200 trace LambdaRuntime : Event's first bytes={"name": "me", "age": 50} aws-request-id=769127502334125 [AWSLambdaRuntime] sending invocation event to lambda handler

[Copilot]

Pull Request Overview

This PR adds trace-level logging functionality to display the raw payload before it's processed by the Lambda handler, porting a feature from Runtime v1 to v2. This enhancement helps debug decoding errors by showing the exact payload received.

Key changes:

• Adds conditional trace logging that shows the first 1KB of raw event payload
• Implements performance optimization by only computing metadata when trace logging is enabled
• Provides truncation indicator when payload exceeds 1KB

[Copilot]

[nitpick] Initialize metadata as nil and then conditionally assign it can be simplified. Consider declaring it as an optional and assigning it directly within the if block, or initialize it as an empty dictionary and conditionally populate it.

Suggested change

	var metadata: Logger.Metadata? = nil
	if logger.logLevel <= .trace,
	let buffer = bytes.getSlice(at: 0, length: min(bytes.readableBytes, 1024))
	{
	metadata = [
	"Event's first bytes": .string(String(buffer: buffer) + (bytes.readableBytes > 1024 ? "..." : ""))
	]
	if logger.logLevel <= .trace,
	let buffer = bytes.getSlice(at: 0, length: min(bytes.readableBytes, 1024))
	{
	let metadata: Logger.Metadata = [
	"Event's first bytes": .string(String(buffer: buffer) + (bytes.readableBytes > 1024 ? "..." : ""))
	]
	logger.trace(
	"Sending invocation event to lambda handler",
	metadata: metadata
	)
	} else {
	logger.trace("Sending invocation event to lambda handler")

[Copilot]

The magic number 1024 appears again here. This should use the same named constant suggested for line 51 to ensure consistency and easier maintenance.

Suggested change

	let buffer = bytes.getSlice(at: 0, length: min(bytes.readableBytes, 1024))
	{
	metadata = [
	"Event's first bytes": .string(String(buffer: buffer) + (bytes.readableBytes > 1024 ? "..." : ""))
	let buffer = bytes.getSlice(at: 0, length: min(bytes.readableBytes, MAX_PREVIEW_BYTES))
	{
	metadata = [
	"Event's first bytes": .string(String(buffer: buffer) + (bytes.readableBytes > MAX_PREVIEW_BYTES ? "..." : ""))

[ktoso]

Looks ok to me :)