Add Duration-type metric param: elapsed time since previous same-label match (ticket 023)

MetricPrimitiveKind::Duration/MetricPrimitive::Duration(i64 ms) is a
new primitive kind. On a metric's Value-role param (ticket 022), kind
Duration is special-cased in apply_metric_to_log: instead of parsing
the captured text, the value is derived as elapsed time since the
previous counter event with the same label set, using the ingestion
timestamp. That "previous event" lookup is tracked via a new
IndexKey::LastMetricEvent/DataValue::LastEventTimestamp bookkeeping
entry rather than Bucket.values itself, because a label set's first
occurrence has nothing to diff against and pushes no Bucket entry at
all (per spec: omit, don't emit a bogus zero) — but it still needs to
be recorded so the *next* occurrence has something to diff against.

Duration values compose with the existing bucket/PromQL surface
unchanged (coerce_to_f64, filter comparisons, histogram()/gauge()).
On any other param (Label-role metric param, or a standalone
ParamDeclaration), Duration kind instead parses captured text as a
literal i64 millisecond count, same as Number. Profile round-trip
(ProfileMetricPrimitiveKind) and the Metrics.tsx declaration form are
updated to match.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/Metrics.tsx

@@ -213,12 +213,13 @@ export function Metrics({ range }: { range: TimeRange | null }) {
                     <option value="string">string</option>
                     <option value="number">number</option>
                     <option value="timestamp">timestamp</option>
+                    <option value="duration">duration</option>
                   </select>
                   <select
                     className="rounded border border-border-soft bg-surface-1 px-1 py-1 text-xs text-text focus:border-border-strong focus:outline-none"
                     value={matcher.role ?? 'label'}
                     onChange={(e) => updateMatcher(i, { role: e.target.value as MetricParamMatcher['role'] })}
-                    title="Label params identify a bucket; at most one Value param becomes the counter's per-event captured value (used by histogram()/gauge())"
+                    title="Label params identify a bucket; at most one Value param becomes the counter's per-event captured value (used by histogram()/gauge()). A Value param of kind 'duration' is derived automatically — elapsed time since the previous match with the same labels — rather than parsed from the capture group."
                   >
                     <option value="label">label</option>
                     <option value="value">value</option>

webapp/src/lib/api.ts

@@ -1,14 +1,17 @@
 /// `MetricPrimitive` is a Rust enum with derived `Serialize`, so its JSON
 /// form is externally tagged, e.g. `{ "String": "info" }`, `{ "Number":
-/// { "Int": 5 } }`, `{ "Timestamp": 1710000000000 }`.
+/// { "Int": 5 } }`, `{ "Timestamp": 1710000000000 }`, `{ "Duration": 1500 }`
+/// (milliseconds, ticket 023).
 export type MetricPrimitive =
   | { String: string }
   | { Number: { Int: number } | { Float: number } }
   | { Timestamp: number }
+  | { Duration: number }
 
 export function metricPrimitiveToDisplayString(value: MetricPrimitive): string {
   if ('String' in value) return value.String
   if ('Timestamp' in value) return String(value.Timestamp)
+  if ('Duration' in value) return `${value.Duration}ms`
   if ('Int' in value.Number) return String(value.Number.Int)
   return String(value.Number.Float)
 }
@@ -27,7 +30,7 @@ export interface ParamDeclaration {
   name: string
   message_match_regex: string
   capture_group: string
-  kind: 'string' | 'number' | 'timestamp'
+  kind: 'string' | 'number' | 'timestamp' | 'duration'
 }
 
 export interface TimelineBucket {
@@ -49,7 +52,7 @@ export type ParamRole = 'label' | 'value'
 export interface MetricParamMatcher {
   param_name: string
   capture_group: string
-  kind: 'string' | 'number' | 'timestamp'
+  kind: 'string' | 'number' | 'timestamp' | 'duration'
   /** Defaults to 'label' server-side when omitted. */
   role?: ParamRole
 }