Metrics rethink: counters as the primitive, histogram()/gauge() derived (ticket 022)

Every metric declaration is now explicitly documented and modeled as
a counter: a match is an occurrence/increment, optionally labeled.
The bucket representation (BucketValue { values: Vec<(Sequence,
Timestamp, MetricPrimitive)> }) already was this; what it lacked was
any way to capture a *value* distinct from the occurrence-count-of-1
marker. Adds RegexToMetricPrimitiveMatcher::role (ParamRole::Label,
the default, or ParamRole::Value — at most one per metric, enforced
at declare time) so a metric can designate one param's captured
number as its per-event value instead of a label.

promql.rs/engine.rs add two new PromQL-evaluable forms built on top
of that value, both consumed as regular PromqlSeriesDto series like
sum()/avg()/rate():
- histogram(metric_name[width]): buckets matching events' values into
  fixed-width buckets, one series per bucket labeled `le`.
- gauge(metric_name): the most-recent value per label combination in
  the queried range.

Existing raw counter queries (metrics with only Label-role params,
sum/avg/rate) are unchanged. Profile files saved before this ticket
still load (role defaults to Label via #[serde(default)]). Metrics.tsx
gains a label/value role picker per param, and the Docs panel explains
the model and the two derived query forms.

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

webapp/src/lib/api.ts

@@ -43,10 +43,15 @@ export interface Stats {
   last_timestamp_ms: number | null
 }
 
+/** Whether a param contributes to a bucket's labels or is its captured value (ticket 022). */
+export type ParamRole = 'label' | 'value'
+
 export interface MetricParamMatcher {
   param_name: string
   capture_group: string
   kind: 'string' | 'number' | 'timestamp'
+  /** Defaults to 'label' server-side when omitted. */
+  role?: ParamRole
 }
 
 export interface MetricDeclaration {