Add React Native perf metrics behind flag

[EdmondChuiHW]

Summary

This PR enables performance metrics to be collected in internal builds of CDT Frontend for performance, reliability, and efficiency.

• Adds new flag for React Native perf metrics
  • Default: off, i.e. no-op for open-source
  • Will only be enabled for internal builds, e.g. via a custom embedderScript.js
• To support setting these configurations before the entry points, embedderScript.js is now a defer script.
  • As type="module" script is defer by definition, the order of these script tags in HTML will be the order of execution per spec.

With the flag enabled, you can use addListener to handle events:

const unsubscribeFn = RNPerfMetrics.Impl.getInstance().addListener(event => console.log(event));

Planned in next PRs

• Add event type definitions
• Add callsites for sendEvent(…) (no-op when flag is false/missing)

Test plan

In both release and default builds:

// in embedderScript.js
globalThis.enableReactNativePerfMetrics = true;
globalThis.enableReactNativePerfMetricsGlobalPostMessage = true;

window.addEventListener('message', event => {
  console.log('got em', event.data.tag);
});

// after rn_inspector.ts
RNPerfMetrics.Impl.getInstance().sendEvent({'tag': 'TestyMcTest'});

Expect console to print TestyMcTest

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.

References

• Similar PR in React DevTools: [DevTools] Add initial APIs for logging instrumentation events under feature flag facebook/react#22276
• Upstream UserMetrics:

  rn-chrome-devtools-frontend/front_end/core/host/UserMetrics.ts

  Line 34 in 9370b35

  export class UserMetrics {

[hoxyq]

Looks good, please see the comment about code paths for Meta's license headers

[hoxyq]

I think you need to add this new file (and other) to

rn-chrome-devtools-frontend/scripts/eslint_rules/lib/check_license_header.js

Lines 69 to 72 in 8c5e07a

	const META_CODE_PATHS = [
	'entrypoints/rn_inspector',
	'panels/rn_welcome',
	];

in order to get correct headers

[EdmondChuiHW]

Ah. Some eslint rule was looking for addEventListener and expects a second param. That crashed so the license linter didn't run either

[hoxyq]

@EdmondChuiHW consider adding front_end/global_typings/react_native.d.ts here as well in one of the next PRs.

[EdmondChuiHW]

Hmm. Looks like .d.ts are excluded somehow. The upstream file here doesn't have license and the linter doesn't complain.

rn-chrome-devtools-frontend/front_end/global_typings/request_idle_callback.d.ts

Line 1 in 9370b35

type RequestIdleCallbackHandle = any;

Added ours to the list for clarity in #18