fix: add hasSubscribers property to TracingChannel

[ssing2]

Fixes #27805

Problem

TracingChannel was missing the hasSubscribers property that exists in Node.js. This made it impossible to check if any channels have subscribers before calling trace methods.

Solution

Added a hasSubscribers getter that returns true if any of the underlying channels have subscribers:

• start
• end
• asyncStart
• asyncEnd
• error

Usage Example

const { tracingChannel } = require('diagnostics_channel');

const trace = tracingChannel('some:random:tracing:channel');

trace.subscribe({
  start (ctx) { console.log('start fired:', ctx.name) },
  end (ctx) { console.log('end fired:', ctx.name) },
  error (ctx) { console.log('error fired:', ctx.error.message) },
});

console.log('has subscribers:', trace.hasSubscribers);
// Before: undefined
// After: true

if (trace.hasSubscribers) {
  trace.traceSync(() => {
    return 'ok:demo';
  }, { name: 'demo' });
}

Changes

• Added hasSubscribers getter to TracingChannel class
• Returns true if any of the 5 channels have subscribers
• Matches Node.js diagnostics_channel API

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Reviewing files that changed from the base of the PR and between e2b0d24 and 3551fe8.

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• src/js/node/diagnostics_channel.ts

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Walkthrough

Added a hasSubscribers getter to the TracingChannel class that aggregates the subscription state across its internal channels (start, end, asyncStart, asyncEnd, error) and exposes it as a public property.

Changes

Cohort / File(s)	Summary
Diagnostic Channel Enhancement src/js/node/diagnostics_channel.ts	Added hasSubscribers computed getter to TracingChannel class that returns true if any of the internal channels (start, end, asyncStart, asyncEnd, error) have active subscribers.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately reflects the main change: adding a hasSubscribers property to TracingChannel to fix the missing feature.
Description check	✅ Passed	The description is comprehensive and follows the template with clear problem statement, solution, and usage example, though verification method is implicitly demonstrated through the usage example.
Linked Issues check	✅ Passed	The PR directly addresses issue #27805 by implementing the hasSubscribers getter that returns true when any of the five channels have subscribers, matching Node.js behavior and enabling the conditional tracing pattern described in the issue.
Out of Scope Changes check	✅ Passed	All changes are scoped to adding the hasSubscribers getter to TracingChannel; no unrelated modifications or out-of-scope code changes are present.

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[robobun]

Thanks for the fix, @ssing2. Main still lacks TracingChannel.hasSubscribers (checked on f426a8e), but #32628 ports Node 26's TracingChannel, which includes this getter and vendors Node's test-diagnostics-channel-tracing-channel-has-subscribers.js. That PR lists Fixes #27805, is green and is still being updated, so closing this one in its favor to keep the diagnostics_channel work in one place. #27805 stays open until #32628 lands.