feat: add diagnostics_channel support for app initialization

[DemianParkhomenko]

Publishes express.initialization channel when an Express app is created,
allowing instrumentation libraries to attach middleware and hooks before
any request arrives.

Route-level tracing (start/end/error per request with matched route pattern)
will be implemented in the router module using TracingChannel API.

Refs: #6353 https://github.com/pillarjs/router/pull/96/changes

[Copilot]

Pull request overview

This PR adds support for Node.js diagnostics_channel to Express, specifically implementing an express.initialization channel that publishes when an Express application is created. This allows instrumentation libraries (like OpenTelemetry) to hook into the app lifecycle before any requests are handled, enabling better observability and monitoring capabilities.

Changes:

• Added a new lib/diagnostics.js module that creates and exports the express.initialization diagnostics channel
• Modified lib/express.js to publish to the initialization channel after app.init() is called
• Added comprehensive tests for the diagnostics channel functionality

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lib/diagnostics.js	New module that creates the express.initialization diagnostics channel
lib/express.js	Imports diagnostics module and publishes initialization event when app is created
test/diagnostics-channel.js	Test suite verifying channel publishes correctly and handles no-subscriber case

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[Copilot]

The codebase convention for top-level requires in lib/ files is to use var rather than const. All other module requires in this file (lines 15-21) use var, and this pattern is consistently followed across all other lib/ files (application.js, request.js, response.js, utils.js, view.js). While const is used in the codebase for destructuring and local variables inside functions, top-level requires should use var for consistency.

Suggested change

	const channels = require('./diagnostics');
	var channels = require('./diagnostics');

[Copilot]

The codebase convention for top-level requires in lib/ files is to use var rather than const. This pattern is consistently followed across all other lib/ files (application.js, request.js, response.js, utils.js, view.js, express.js). While const is used in the codebase for destructuring and local variables inside functions, top-level requires should use var for consistency with the rest of the codebase.

Suggested change

	const dc = require('node:diagnostics_channel');
	const initialization = dc.channel('express.initialization');
	var dc = require('node:diagnostics_channel');
	var initialization = dc.channel('express.initialization');