♻️ Move global monitor functions to js-core

[BenoitZugmeyer]

Motivation

As more utilities move from browser-core to js-core (starting with httpRequest), several shared functions need access to the monitor error-collection helpers. These are used pervasively as bare globals — passed to addEventListener, setTimeout, Observable subscriptions, etc. — so they must remain globally available rather than threaded through every call site. Hosting the global monitor in js-core lets shared code there use it directly and unblocks further utility migrations.

When several SDKs share the same js-core module instance, only the first SDK to init receives internal-error telemetry from the global monitor (first-wins). This is a deliberate trade-off for simplicity over per-SDK attribution, which can be revisited in the future.

Alternative considered

I investigated how we could have one "Monitor" instance per SDK using js-core, and pass it implicitly to asynchronous functions via monitor wrappers. But I hit a road block, as there is no good way to apply this strategy to async functions: awaiting a promise creates a new microtask that cannot be wrapped in anyway. As I want to move forward on js-core, I chose the simple way, expose monitor global functions that anyone can use. SDKs can still create their own monitor functions if they want to.

Changes

• Expose a global monitor from @datadog/js-core/monitor (monitor, callMonitored, monitored, monitorError) alongside the existing createMonitor factory.
• Add startMonitorErrorCollection(cb) (first-wins, returns false if already claimed) and stopMonitorErrorCollection() (production detach path) to manage the error-collection sink.
• Remove the global monitor implementation and its re-export from browser-core; all consumers (browser-core internals, browser-rum-core, browser-rum, browser-logs, and framework integrations) now import monitor helpers directly from @datadog/js-core/monitor.
• Export monitorDisplay from @datadog/js-core/monitor so tests can spy on the monitor's debug output (the global monitor no longer shares a display instance with browser-core).
• Document the multi-SDK first-wins limitation in the startMonitorErrorCollection JSDoc.

Test instructions

• In the sandbox (yarn dev), load the SDK and trigger an internal error (e.g. misconfigure an endpoint so a monitored callback throws). Confirm the error is captured in telemetry and no uncaught exception surfaces to the page.
• Enable debug mode (DD_RUM._setDebug(true) / DD_LOGS._setDebug(true)) and trigger a monitored error; confirm a [MONITOR] error is logged to the console with the Datadog SDK: prefix.
• Load both RUM and Logs on the same page; confirm internal errors are attributed to the first SDK that initialised and the second does not hijack the sink.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[datadog-datadog-prod-us1-2]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 90.91%
• Overall Coverage: 77.13% (+0.10%)

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[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	181.52 KiB	181.56 KiB	+42 B	+0.02%	✅
Rum Profiler	8.43 KiB	8.43 KiB	-1 B	-0.01%	✅
Rum Recorder	22.31 KiB	22.31 KiB	-1 B	-0.00%	✅
Logs	57.63 KiB	57.67 KiB	+44 B	+0.07%	✅
Rum Salesforce	N/A	139.64 KiB	N/A	N/A	N/A
Rum Slim	139.59 KiB	139.63 KiB	+44 B	+0.03%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	202.39 KiB	N/A	N/A	N/A
Rum-shopify Profiler	N/A	8.43 KiB	N/A	N/A	N/A
Rum-shopify Recorder	N/A	3.72 KiB	N/A	N/A	N/A

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Document the decorator parameters

issue: Add @param documentation for the decorator target, property name, and descriptor. This newly exported stable alias exposes parameters named _ and __, so consumers cannot infer what they represent, while the package’s non-negotiable export-documentation rule requires parameter descriptions when names are not self-explanatory.

AGENTS.md reference: packages/js-core/AGENTS.md:L24-L31

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