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

src/data_value.rs

@@ -9,6 +9,23 @@ pub struct RegexToMetricPrimitiveMatcher {
     pub param_name: String,
     pub capture_group: String,
     pub kind: MetricPrimitiveKind,
+    /// Ticket 022: whether this param contributes to the bucket's `labels`
+    /// (the default) or is the bucket's per-event captured `value` (at most
+    /// one `Value`-role param per metric — `declare_metric` rejects more
+    /// than one). `#[serde(default)]` so profile files saved before this
+    /// field existed still load, with every param defaulting to `Label` —
+    /// i.e. unchanged behavior (the fixed occurrence-count-of-1 value).
+    #[serde(default)]
+    pub role: ParamRole,
+}
+
+/// See [`RegexToMetricPrimitiveMatcher::role`].
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "lowercase")]
+pub enum ParamRole {
+    #[default]
+    Label,
+    Value,
 }
 
 /// `rename_all = "lowercase"` matches every other `kind`-style enum exposed