Add gauge/counter semantics to metric declarations (ticket 097)

Metrics previously only declared a data type (String/Number/...), so
rate() could be applied to a point-in-time gauge reading and sum()/avg()
could be applied directly to a cumulative counter reading with no guard
against either. Adds a MetricSemantics enum (Gauge, default; Counter)
threaded through DeclareMetricRequest, the engine's MetricDeclaration
data value, profile persistence, the API/MCP declaration routes, and the
Metrics panel's declaration form, then uses it in evaluate_promql to
reject rate() on a declared gauge and bare sum()/avg() on a declared
counter (suggesting rate() first). The guard only applies to metrics
with a captured value_param — bare occurrence-count metrics are always
safe under both operations, so existing dashboards are unaffected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

src/engine/snapshot/mod.rs

@@ -18,7 +18,9 @@ use crate::consts::{
     MAX_AUTO_DECLARED_PARAMS, MAX_NEW_AUTO_PARAMS_PER_LINE, OTTYL_RESERVED_PARAM_NAMES,
     SORT_HELPER_INDEX_PURGE_AFTER,
 };
-use crate::engine::data_value::{DataValue, MetricPrimitiveKind, ParamDeclaration};
+use crate::engine::data_value::{
+    DataValue, MetricPrimitiveKind, MetricSemantics, ParamDeclaration,
+};
 use crate::engine::filter_parser::{self, CompareOp, FilterExpr, FilterValue};
 use crate::engine::index_key::IndexKey;
 use crate::engine::message::{EngineQuery, EngineResponse};
@@ -53,6 +55,7 @@ type MetricDeclarationRow<'a> = (
     Option<&'a str>,
     Option<&'a BTreeMap<String, String>>,
     Option<&'a BTreeMap<String, String>>,
+    MetricSemantics,
 );
 
 /// Per-label-group point series, keyed by the group's label set, before merging into one
@@ -91,6 +94,7 @@ impl EngineSnapshot {
                         value_param,
                         matched_labels,
                         unmatched_labels,
+                        semantics,
                     },
                 ) => Some((
                     name.as_str(),
@@ -99,6 +103,7 @@ impl EngineSnapshot {
                     value_param.as_deref(),
                     matched_labels.as_ref(),
                     unmatched_labels.as_ref(),
+                    *semantics,
                 )),
                 _ => None,
             })
@@ -420,6 +425,7 @@ impl EngineSnapshot {
                 value_param: request.value_param,
                 matched_labels: request.matched_labels,
                 unmatched_labels: request.unmatched_labels,
+                semantics: request.semantics,
             },
         );
     }
@@ -618,7 +624,7 @@ impl EngineSnapshot {
         let metrics: Vec<SeededMetric> = self
             .metric_declarations_range()
             .map(
-                |(name, _, label_params, value_param, matched_labels, unmatched_labels)| {
+                |(name, _, label_params, value_param, matched_labels, unmatched_labels, _)| {
                     (
                         name.to_string(),
                         label_params.to_vec(),
@@ -708,6 +714,32 @@ impl EngineSnapshot {
             _ => None,
         }
     }
+
+    pub(crate) fn declared_semantics(&self, metric_name: &str) -> Option<MetricSemantics> {
+        match self.index.get(&IndexKey::MetricDeclaration {
+            name: metric_name.to_string(),
+        }) {
+            Some(DataValue::MetricDeclaration { semantics, .. }) => Some(*semantics),
+            _ => None,
+        }
+    }
+
+    /// Whether `metric_name` captures a per-event value (rather than just counting matching log
+    /// lines). The gauge/counter semantics guard in `evaluate_promql` only applies to metrics
+    /// with a captured value: a bare occurrence count is always safe to both `rate()` (events per
+    /// window) and `sum()`/`avg()` (total/average occurrences) since each event contributes
+    /// exactly one point, unlike a captured reading that might itself be a cumulative total.
+    fn declares_value_param(&self, metric_name: &str) -> bool {
+        matches!(
+            self.index.get(&IndexKey::MetricDeclaration {
+                name: metric_name.to_string(),
+            }),
+            Some(DataValue::MetricDeclaration {
+                value_param: Some(_),
+                ..
+            })
+        )
+    }
 }
 
 // --- params: declaring and resolving param values ---
@@ -1068,6 +1100,33 @@ impl EngineSnapshot {
                 parsed.metric_name
             ));
         }
+        // A metric with no captured value (just occurrence counts of matching log lines) is
+        // always safe under both rate() and sum()/avg(): each event contributes exactly one
+        // point, so there's no cumulative-total to misinterpret. The guard below only matters
+        // once a metric captures a per-event value via `value_param`.
+        let declared_semantics = self
+            .declares_value_param(&parsed.metric_name)
+            .then(|| self.declared_semantics(&parsed.metric_name))
+            .flatten();
+        if matches!(parsed.agg, Some(AggOp::Rate))
+            && declared_semantics == Some(MetricSemantics::Gauge)
+        {
+            return Err(format!(
+                "cannot rate() \"{}\": it's declared as a gauge, not a counter; gauges are point-in-time \
+                 readings, so rate() over them isn't meaningful — use it bare or wrap it in gauge()",
+                parsed.metric_name
+            ));
+        }
+        if matches!(parsed.agg, Some(AggOp::Sum) | Some(AggOp::Avg))
+            && !parsed.inner_rate
+            && declared_semantics == Some(MetricSemantics::Counter)
+        {
+            return Err(format!(
+                "cannot sum/avg \"{}\" directly: it's declared as a counter, so summing/averaging its raw \
+                 values double-counts the running total — wrap it in rate() first, e.g. sum(rate({}[5m]))",
+                parsed.metric_name, parsed.metric_name
+            ));
+        }
 
         let mut series_points: BTreeMap<BTreeMap<String, String>, Vec<(i64, f64)>> =
             BTreeMap::new();