Document EventCounts API

[Elchi3]

Description

This PR documents a neat little API that allows you to get counts for events via performance.eventCounts.

EventCounts is basically a Map but readonly. I don't think we should have sub pages there. I pointed to the Map docs instead.

The list of event types that are possible to count here are listed on https://developer.mozilla.org/en-US/docs/Web/API/PerformanceEventTiming. That list looks a bit lost but I want to create a separate PR to properly document PerformanceEventTiming. Stay tuned ;)

Motivation

This has been supported in Firefox and Chrome for a while.
Also contributes to the request made by the Performance WG to update MDN perf API docs, see openwebdocs/project#62

Additional details

It seems that https://developer.mozilla.org/en-US/docs/Web/API/MIDIInputMap is some prior art on documenting readonly maplike interfaces, but I think that page is quite thin.

Related issues and pull requests

I made a BCD PR to fix spec_urls, mdn_urls and standard statuses, see mdn/browser-compat-data#17989

[github-actions]

Preview URLs

• /en-US/docs/Web/API/EventCounts
• /en-US/docs/Web/API/Performance/eventCounts
• /en-US/docs/Web/API/Performance

Flaws (1)

Note! 2 documents with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/API/Performance
Title: Performance
Flaw count: 1

• macros:
  • /en-US/docs/Web/API/Window/performance redirects to /en-US/docs/Web/API/performance_property

[teoli2003]

This is clearly better. We can discuss how to document readonly maplike<> in the future (as well as setlike<>: I think we should document the pages as they are not real maps, and it can be confusing for a beginner developer. (Though we should solve the extra work by having a working template generator)

But no need to hold this now, as it is much better than before.