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/profile.rs

@@ -40,7 +40,7 @@ use std::time::{SystemTime, UNIX_EPOCH};
 
 use serde::{Deserialize, Serialize};
 
-use crate::data_value::{MetricPrimitiveKind, RegexToMetricPrimitiveMatcher};
+use crate::data_value::{MetricPrimitiveKind, ParamRole, RegexToMetricPrimitiveMatcher};
 use crate::engine::{DeclareMetricRequest, DeclareParamRequest, EngineHandle, EngineMessage};
 use crate::settings::Settings;
 use tokio::sync::oneshot;
@@ -119,6 +119,10 @@ pub struct MetricParamRecord {
     pub param_name: String,
     pub capture_group: String,
     pub kind: ProfileMetricPrimitiveKind,
+    /// See `ParamRole`. `#[serde(default)]` so profiles saved before ticket
+    /// 022 still load, with every param defaulting to `Label`.
+    #[serde(default)]
+    pub role: ParamRole,
 }
 
 /// A stored metric declaration, matching `DeclareMetricRequest`'s shape (ticket 002).
@@ -141,6 +145,7 @@ impl From<&DeclareMetricRequest> for MetricDeclarationRecord {
                     param_name: matcher.param_name.clone(),
                     capture_group: matcher.capture_group.clone(),
                     kind: matcher.kind.into(),
+                    role: matcher.role,
                 })
                 .collect(),
         }
@@ -159,6 +164,7 @@ impl From<&MetricDeclarationRecord> for DeclareMetricRequest {
                     param_name: param.param_name.clone(),
                     capture_group: param.capture_group.clone(),
                     kind: param.kind.into(),
+                    role: param.role,
                 })
                 .collect(),
         }
@@ -708,6 +714,7 @@ mod tests {
                 param_name: param_name.to_string(),
                 capture_group: capture_group.to_string(),
                 kind: MetricPrimitiveKind::Number,
+                role: ParamRole::Label,
             }],
         }
     }