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/components/Metrics.tsx

@@ -214,6 +214,15 @@ export function Metrics({ range }: { range: TimeRange | null }) {
                     <option value="number">number</option>
                     <option value="timestamp">timestamp</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())"
+                  >
+                    <option value="label">label</option>
+                    <option value="value">value</option>
+                  </select>
                   <button
                     type="button"
                     onClick={() => removeMatcher(i)}