Detect and warn if duplicate lambda libraries are detected

[nhulston]

What does this PR do?

Asynchronously detects if the Lambda library is installed both in node_modules and as a Lambda library. If so, it logs a warning.

Motivation

It's common for users to install the Lambda layers as instructed by the documentation, but then then see import errors in their code, causing them to run npm install datadog-lambda-js. Now they will have the Lambda library installed in two places, which will (1) increase package size and cold start duration, and (2) break custom metrics.

We've had many customers open support tickets recently about broken metrics caused by this issue, so this PR aims to avoid this problem in the future.

Testing Guidelines

Manual tests: logs a warning as expected when installed in both places, and doesn't log any warning if only installed in the layer or node_modules.

Added unit tests.

Additional Notes

This works by using fs and checking if the paths exist. I'm assuming the paths will always be:

• Layer: '/opt/nodejs/node_modules/datadog-lambda-js'
• Node modules: path.join(process.cwd(), 'node_modules/datadog-lambda-js')

We could use process.env.NODE_PATH and search for datadog-lambda-js/package.json in the paths. However, my solution is simpler and works perfectly fine. It is also faster than relying on node path because we don't have to parse the node path.

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[nhulston]

I put this under the datadog() wrapper, but I'm not sure if this is the best place to make this call. If there's somewhere that's better, I'm open to suggestions!

[nhulston]

Also, this code might be relevant, so wanted to share for reviewers:

datadog-lambda-js/src/trace/tracer-wrapper.ts

Lines 28 to 44 in f05c600

	export class TracerWrapper {
	private tracer: any;
	constructor() {
	try {
	// Try and use the same version of the tracing library the user has installed.
	// This handles edge cases where two versions of dd-trace are installed, one in the layer
	// and one in the user's code.
	const path = require.resolve("dd-trace", { paths: ["/var/task/node_modules", ...module.paths] });
	this.tracer = require(path);
	return;
	} catch (err) {
	if (err instanceof Object || err instanceof Error) {
	logDebug("Couldn't require dd-trace from main", err);
	}
	}
	}

Note that this code only loads in one version of dd-trace, not to be confused with the datadog-lambda-js; this PR detects duplicate installations of datadog-lambda-js

[duncanista]

Wonder if there's a way for us to use similar logic like this one 🤔