Perf: add support for launch ID

[EdmondChuiHW]

Summary

TLDR: We want to keep a stable ID for each launch so we can join CDT sessions.

Open source: no-op as perf metrics is only applicable for internal builds. See #16

Please also see the internal RFC: https://www.internalfb.com/diff/D55164646

This launch ID is read from the query param; thus remains the same for each CDT refresh, either by pressing F5 or clicking "Reconnect DevTools":

rn-chrome-devtools-frontend/front_end/ui/legacy/RemoteDebuggingTerminatedScreen.ts

Line 47 in 9370b35

const button = createTextButton(i18nString(UIStrings.reconnectDevtools), () => window.location.reload());

Why

Since modern web standards relies on visibilitychange to signal the end of a session, we need to account for users showing and hiding the window multiple times. Closing the window is indistinguishable from hiding the window under this model.

• MDN: Sending analytics at the end of a session

This means each time the window is hidden, we treat this as one completed CDT session and submit end-of-session perf metrics as if the window is closed.

If the user was only hiding the window and came back later, we need a way to de-dup the subsequent events.

Therefore, we need to keep a stable ID across refreshes and visibilitychange events. For each launch, e.g. from iOS, Android, or Metro, a launch ID is generated in the internal version of Metro. See internal diff

We'll then treat all events with the same launch ID as a single session in our metrics analytics.

Pending further discussion, we can also use this launch ID for Metro and CDP backend to attribute perf metrics outside of CDT.

Stack

Stacked on #26

Depended by #28

Preview this PR in isolation at https://github.com/EdmondChuiHW/rn-chrome-devtools-frontend2/pull/1/files

Test plan

Non-null launchId in query param doesn't crash when launched from iOS, Android, and Metro (internal)

Null launchId query param: no-op (for open-source)

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.

[huntie]

Different casing here compared to L23? 🤔

[EdmondChuiHW]

Yeah one is static/constructor. One is the global instance. Following the pattern of the upstream UserMetrics:

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

Lines 39 to 48 in 9370b35

	/** @constructor */
	Host.UserMetrics = HostModule.UserMetrics.UserMetrics;
	Host.UserMetrics._PanelCodes = HostModule.UserMetrics.PanelCodes;
	/** @enum {number} */
	Host.UserMetrics.Action = HostModule.UserMetrics.Action;
	/** @type {!Host.UserMetrics} */
	Host.userMetrics = HostModule.userMetrics;

Can move the first call to become an instance method instead. At first it was a single function, but this repo enforces a namespace-style import so that's why it was put into RNPerfMetrics "static".

[huntie]

LGTM :)