Add Duration-type metric param: elapsed time since previous same-label match (ticket 023)

MetricPrimitiveKind::Duration/MetricPrimitive::Duration(i64 ms) is a
new primitive kind. On a metric's Value-role param (ticket 022), kind
Duration is special-cased in apply_metric_to_log: instead of parsing
the captured text, the value is derived as elapsed time since the
previous counter event with the same label set, using the ingestion
timestamp. That "previous event" lookup is tracked via a new
IndexKey::LastMetricEvent/DataValue::LastEventTimestamp bookkeeping
entry rather than Bucket.values itself, because a label set's first
occurrence has nothing to diff against and pushes no Bucket entry at
all (per spec: omit, don't emit a bogus zero) — but it still needs to
be recorded so the *next* occurrence has something to diff against.

Duration values compose with the existing bucket/PromQL surface
unchanged (coerce_to_f64, filter comparisons, histogram()/gauge()).
On any other param (Label-role metric param, or a standalone
ParamDeclaration), Duration kind instead parses captured text as a
literal i64 millisecond count, same as Number. Profile round-trip
(ProfileMetricPrimitiveKind) and the Metrics.tsx declaration form are
updated to match.

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

src/index_key.rs

@@ -33,6 +33,17 @@ pub enum IndexKey {
         metric_name: String,
         labels: BTreeMap<String, MetricPrimitive>,
     },
+    /// Ticket 023: internal bookkeeping (never returned by a query) tracking
+    /// the timestamp of the most recent counter event for one metric+label
+    /// combination, independent of whether that event actually pushed a
+    /// `Bucket` value — a `Duration`-kind `Value` param's *first* occurrence
+    /// of a label set has no previous event to diff against and pushes no
+    /// `Bucket` entry, but must still be recorded here so the *next*
+    /// occurrence can compute its elapsed time.
+    LastMetricEvent {
+        metric_name: String,
+        labels: BTreeMap<String, MetricPrimitive>,
+    },
     /// Marker for the end of the whole range. Nothing sorts after this.
     End,
 }