[mwawacek]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit messages follow the Angular convention, as commitlint checks them: https://github.com/nestjs/observe/blob/master/.commitlintrc.json
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe: performance fix

What is the current behavior?

Every observe on a summary from TracerService re-encodes it into the outgoing batch (upsertCustomMetric), and encoding a summary sorts a copy of its sample once for each of p50, p95 and p99. The sample keeps at most 2,048 observations per label, so the cost of one observe grows with the number of observations since the last flush until the sample is full, and then stays at that level:

observations in the window	one observe before	after
500	147 µs	< 1 µs
2,048	964 µs	< 1 µs
5,000	985 µs	< 1 µs
20,000	977 µs	< 1 µs

That time is spent on the main thread, inside the request that calls observe. Measured on Node 22 against the built package, timing the last 200 of n observations of one summary in a single window:

const summary = new Summary("request.duration");
const buffer = new ObserveAgentSharedBuffer({ serviceId: "svc", forwardLogs: true });
for (let i = 0; i < n; i++) {
  summary.observe(Math.random() * 100);
  buffer.upsertCustomMetric(summary);
}
buffer.encodeAndWrite();

What is the new behavior?

A summary observed into the batch is remembered and encoded once, when the batch is written in encodeAndWrite (about 1-3 ms per batch in the runs above, up from about 0.1 ms). observe no longer sorts. Counters and gauges are buffered as before. The values that reach the collector are unchanged, since the summary is encoded from the same window, just once. Only the order within custom changes: summaries now come at the end of the array.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information