feat: Event-time based aggregation support

[adiwab]

#14015

Summary

1. Event-time handling (absolute metrics in Auto and Latest/Diff):

   • Now compares event timestamps and only replaces the stored value if the new sample is newer (or equal).
   • Prevents late/out-of-order samples from rolling back bucket values (Event clock only).

2. Processing-time path:

   • Absolute metrics in Auto and Latest/Diff are still replaced by the last arriving sample (no event-timestamp check yet).

3. Other aggregation modes preserved:

   • Sum, Count for incremental metrics unchanged.
   • Max, Min, Mean, Stdev continue to aggregate multiple samples per bucket.
   • Diff emits the difference between the current and previous bucket values.

4. New configuration options:

   • clock: choose bucket alignment by Processing (Vector wall clock) or Event (event timestamps).
   • allowed_lateness_ms: wait time for late or out-of-order samples in event-time mode (default: 120s).
   • emit_ts: set output timestamp to BucketStart or BucketEnd.

Important Notes

• This is a draft implementation.
• It requires thorough testing (unit and integration) to verify correctness across modes, especially with late or out-of-order samples.
• There may be room for further optimization (e.g., reducing allocations, refining how late samples are handled).

Vector configuration

prom_aggregate:
type: aggregate
inputs:
- prometheus
interval_ms: 60000
mode: Latest
clock: Event
allowed_lateness_ms: 60000

How did you test this PR?

I built the changes locally (win11 - Build 26100) and verified the results by sending metrics into InfluxDB and inspecting them in Grafana dashboards.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Related: Support metrics aggregation windowing by event timestamp #14015

Notes

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[pront]

Hi @adiwab, the PR title feat! indicates this a breaking change (https://www.conventionalcommits.org/en/v1.0.0/), but I don't think this is true.