# 023 — Duration-type parameter

## Summary

Support a `Duration` parameter kind: its value is the time elapsed since the
previous trigger of the *same* metric with the *same* labels/params, measured
using whichever timestamp field the user has configured (ingestion time by
default, or a param-derived timestamp).

## Scope

- New `MetricPrimitive`/param-kind variant for durations (e.g. milliseconds as an
  `i64`, consistent with how `Timestamp` is already stored).
- Derivation rule: for a metric match with a given label set, look up that same
  label set's previous counter event ([022](022-metrics-counters-rethink.md)) and
  compute `this_event_time - previous_event_time` using the configured time
  source (ingestion timestamp, or a declared timestamp param per the README's
  "deriving a timestamp different from the ingested timestamp" note).
- First occurrence of a given label set has no previous event — define and
  document the behavior (e.g. omit the duration value, or emit `null`/zero;
  omitting is likely correct so it doesn't pollute histograms with a bogus zero).
- Expose duration values the same way other derived/counter data is queried
  (bucket lookup, PromQL, Metrics UI) so it composes with existing dashboards.

## Acceptance criteria

- Two log lines matching the same metric+labels, seconds apart, produce a
  duration value equal to that elapsed time on the second occurrence.
- The first occurrence of a given label set produces no duration value (not a
  zero or garbage value).
- Duration values are queryable via the same bucket/PromQL surface as other
  metric data.

## Depends on

- [022 — Metrics rethink](022-metrics-counters-rethink.md) (durations are computed
  between successive counter events of the same metric).