Add Metrics API: declare/list params and metrics, query bucket values (ticket 004)

Exposes param/metric declarations and derived bucket data over HTTP:
POST /api/params and POST /api/metrics validate and declare against the
engine, persisting into the active profile (if any) so declarations survive
a restart; GET /api/metrics lists declarations; GET /api/metrics/:name
queries bucket values with label, time-range, sequence, and limit filters.
Adds a new EngineQuery::BucketValues variant that layers this filtering on
top of ticket 003's bucket_values helper.

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

src/metric_primitive.rs

@@ -1,6 +1,7 @@
 use crate::timestamp::Timestamp;
 use serde::Serialize;
 use std::cmp::Ordering;
+use std::fmt;
 
 /// A simple, self-describing value used by metrics/labels without pulling in JSON.
 #[allow(dead_code)]
@@ -42,6 +43,21 @@ impl Ord for MetricPrimitive {
     }
 }
 
+/// Textual form used for label-filter comparisons over the metrics API
+/// (`GET /api/metrics/:name?labels=foo=bar`): query-string values are plain
+/// strings, so a label filter is compared against this formatted form rather
+/// than trying to parse the filter value into a typed `MetricPrimitive`.
+impl fmt::Display for MetricPrimitive {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            MetricPrimitive::String(s) => write!(f, "{s}"),
+            MetricPrimitive::Number(Number::Int(n)) => write!(f, "{n}"),
+            MetricPrimitive::Number(Number::Float(n)) => write!(f, "{n}"),
+            MetricPrimitive::Timestamp(ts) => write!(f, "{}", ts.as_millis()),
+        }
+    }
+}
+
 /// Numbers are kept as a tagged int/float pair so exact integers stay exact.
 #[allow(dead_code)]
 #[derive(Debug, Clone, Copy, Serialize)]