[bengl]

flush() used to start an export and return nothing, so there was no way to wait for spans to reach the collector. Explicit exits skip the beforeExit flush, and so does a serverless platform that freezes the process after each invocation. Now flush() returns a promise:

process.on('SIGTERM', async () => {
  await otel.flush();
  process.exit(0);
});

What the promise covers. It resolves once the spans flush() took from the buffer, and every export already in flight, are done, whether they reached the collector or not. A flush() called while an export is in flight waits for that export too, so awaiting the latest call is enough. Spans that end after the call belong to a later flush.

It never rejects. Failed exports are already reported as OTelExportWarning warnings. A rejection would be unhandled in every caller that ignores the result, including microtel's own timer, buffer-full, and beforeExit flushes.

How an export settles. Each export gets a promise, kept in a set while its request is open. It settles on whichever comes first: the response ending, a response or request error, or the exportTimeout deadline, which destroys the request. An export that throws while sending destroys its request, so neither its socket nor its deadline outlives it.

Using it.

• benchmark/export.bench.js awaits its flushes instead of counting requests at the collector, and drops its watchdog. A short exportTimeout keeps a stalled export from hanging a sample. Each sample now ends when the last response does, rather than when the collector finishes reading the last request, so its numbers are a little lower than main's.
• Several tests waited a fixed delay(100) after flush() to see whether an export happened. They now await flush() itself.
• The README documents the promise, shows a shutdown handler, and updates the note about explicit exits.

Not taken.

• Resolving with counts of what was sent or dropped. It can be added later without breaking anything.
• A timeout option for flush(). exportTimeout already bounds every export.

Tests. test/test-flush-promise.js covers:

• the batch already being at the collector when the promise resolves
• resolving right away with nothing to export, and before init()
• resolving, not rejecting, when the collector is down, never answers, or resets mid-response
• a second flush() waiting for an export still in flight
• a send that throws, where the child must exit well before its 60 second exportTimeout

Each of these breaks fails the file: not waiting for exports in flight, settling by rejecting, resolving before the export ends, a response or request error that never settles, and a failed send that leaves its request open.

[AbhiPrasad]

if we do it async should we let folks pass in a AbortController?

also maybe we should have a max timeout (allow for it to be configured?)

[bengl]

@AbhiPrasad wanna do those in a separate PR?

[AbhiPrasad]

separate pr sgtm