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/config/profile.rs

@@ -13,7 +13,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::config::Settings;
 use crate::consts::{DEFAULT_PROFILE_NAME, SEEDED_METRIC_LABEL, SEEDED_METRIC_NAME};
-use crate::engine::data_value::MetricPrimitiveKind;
+use crate::engine::data_value::{MetricPrimitiveKind, MetricSemantics};
 use crate::engine::{DeclareMetricRequest, DeclareParamRequest, EngineHandle, EngineMessage};
 use tokio::sync::oneshot;
 
@@ -122,6 +122,8 @@ pub struct MetricDeclarationRecord {
     pub matched_labels: Option<std::collections::BTreeMap<String, String>>,
     #[serde(default)]
     pub unmatched_labels: Option<std::collections::BTreeMap<String, String>>,
+    #[serde(default)]
+    pub semantics: MetricSemantics,
     #[serde(default, rename = "params", skip_serializing_if = "Vec::is_empty")]
     legacy_params: Vec<LegacyMetricParamRecord>,
 }
@@ -135,6 +137,7 @@ impl From<&DeclareMetricRequest> for MetricDeclarationRecord {
             value_param: request.value_param.clone(),
             matched_labels: request.matched_labels.clone(),
             unmatched_labels: request.unmatched_labels.clone(),
+            semantics: request.semantics,
             legacy_params: Vec::new(),
         }
     }
@@ -149,6 +152,7 @@ impl From<&MetricDeclarationRecord> for DeclareMetricRequest {
             value_param: record.value_param.clone(),
             matched_labels: record.matched_labels.clone(),
             unmatched_labels: record.unmatched_labels.clone(),
+            semantics: record.semantics,
         }
     }
 }
@@ -295,6 +299,7 @@ impl MetricDeclarationRecord {
                 SEEDED_METRIC_LABEL.to_string(),
                 "other".to_string(),
             )])),
+            semantics: MetricSemantics::default(),
             legacy_params: Vec::new(),
         }
     }
@@ -741,6 +746,7 @@ mod tests {
             unmatched_labels: None,
             label_params: vec![param_name.to_string()],
             value_param: None,
+            semantics: MetricSemantics::default(),
         }
     }