perf: skip snapshot rebuild in mergeDuplicates when names are unique

[david-mollitor-db]

What

TextFormatUtil.mergeDuplicates currently only short-circuits when there is a single snapshot. For every scrape with two or more metric families it unconditionally builds a LinkedHashMap, an ArrayList per group, a MetricSnapshots.Builder (with its own list + set) and a freshly sorted MetricSnapshots — even when there is nothing to merge, which is the common case.

Since MetricSnapshots is always sorted by prometheus name (see its constructor), any duplicate names are adjacent. This adds a single allocation-free pass to detect duplicates; when there are none, the input is returned unchanged.

boolean hasDuplicates = false;
for (int i = 1; i < metricSnapshots.size(); i++) {
  if (metricSnapshots.get(i).getMetadata().getPrometheusName()
      .equals(metricSnapshots.get(i - 1).getMetadata().getPrometheusName())) {
    hasDuplicates = true;
    break;
  }
}
if (!hasDuplicates) {
  return metricSnapshots;
}

The merge path for actual duplicates is unchanged.

Why

It removes the map / per-group list / builder / re-sort allocations on every scrape that has no duplicate metric names. Measured on a JMH benchmark of a histogram-heavy scrape (collect + serialize to the Prometheus text format), allocation on the serialize path dropped by ~880 B/op, scaling with the number of metric families.

Correctness

Output is byte-identical. The existing exposition-format tests pass, including DuplicateNamesExpositionTest, which exercises the merge path with real duplicate names.

This pull request and its description were written by Isaac.