Extract shared declare/update-metric backfill and auto-param insertion helpers

declare_metric and update_metric duplicated the resolve-params +
backfill_metric + insert-MetricDeclaration sequence; extracted into
resolve_backfill_and_store_metric so the declare-vs-rename difference
is expressed only in caller-side logic. Also extracted
auto_declare_params's per-candidate insertion body into
try_insert_auto_param so the candidate-building and insertion steps
read as separate concerns.

Reimplemented directly against current main (ticket 085) after the
original worktree agent's branch was based on a stale pre-consolidation
snapshot module layout that no longer matches main.

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

src/engine/snapshot/mod.rs

@@ -330,11 +330,18 @@ impl EngineSnapshot {
     pub(crate) fn declare_metric(&mut self, request: DeclareMetricRequest) -> Result<(), String> {
         let re = self.validate_metric_request(&request)?;
         self.metric_regex_cache.insert(request.name.clone(), re.clone());
-        let resolved_params = self.resolved_param_declarations();
+        self.resolve_backfill_and_store_metric(request, &re);
+        Ok(())
+    }
 
+    /// Shared by `declare_metric` and `update_metric`: resolves param declarations, backfills
+    /// the metric's buckets from existing log history, then writes the `MetricDeclaration` entry.
+    /// Callers differ only in what happens before this (rename bookkeeping for updates).
+    fn resolve_backfill_and_store_metric(&mut self, request: DeclareMetricRequest, re: &Regex) {
+        let resolved_params = self.resolved_param_declarations();
         self.backfill_metric(
             &request.name,
-            &re,
+            re,
             &request.label_params,
             request.value_param.as_deref(),
             &resolved_params,
@@ -342,7 +349,7 @@ impl EngineSnapshot {
             request.unmatched_labels.as_ref(),
         );
         self.index.insert(
-            IndexKey::MetricDeclaration { name: request.name },
+            IndexKey::MetricDeclaration { name: request.name.clone() },
             DataValue::MetricDeclaration {
                 message_match_regex: request.message_match_regex,
                 label_params: request.label_params,
@@ -351,7 +358,6 @@ impl EngineSnapshot {
                 unmatched_labels: request.unmatched_labels,
             },
         );
-        Ok(())
     }
 
     pub(crate) fn validate_update_metric(
@@ -372,7 +378,6 @@ impl EngineSnapshot {
 
     pub(crate) fn update_metric(&mut self, existing_name: &str, request: DeclareMetricRequest) -> Result<(), String> {
         let re = self.validate_update_metric(existing_name, &request)?;
-        let resolved_params = self.resolved_param_declarations();
 
         self.clear_metric_data(existing_name);
         if request.name != existing_name {
@@ -381,25 +386,7 @@ impl EngineSnapshot {
         }
         self.metric_regex_cache.insert(request.name.clone(), re.clone());
 
-        self.backfill_metric(
+        self.resolve_backfill_and_store_metric(request, &re);
-            &request.name,
-            &re,
-            &request.label_params,
-            request.value_param.as_deref(),
-            &resolved_params,
-            request.matched_labels.as_ref(),
-            request.unmatched_labels.as_ref(),
-        );
-        self.index.insert(
-            IndexKey::MetricDeclaration { name: request.name.clone() },
-            DataValue::MetricDeclaration {
-                message_match_regex: request.message_match_regex,
-                label_params: request.label_params,
-                value_param: request.value_param,
-                matched_labels: request.matched_labels,
-                unmatched_labels: request.unmatched_labels,
-            },
-        );
         Ok(())
     }
 
@@ -698,9 +685,18 @@ impl EngineSnapshot {
             if new_this_line >= MAX_NEW_AUTO_PARAMS_PER_LINE || total >= MAX_AUTO_DECLARED_PARAMS {
                 break;
             }
+            if self.try_insert_auto_param(candidate, &mut total) {
+                new_this_line += 1;
+            }
+        }
+    }
+
+    /// Inserts `candidate` as an auto-derived `ParamDeclaration` unless a param with that name
+    /// is already declared, bumping `*total` and returning whether it inserted.
+    fn try_insert_auto_param(&mut self, candidate: AutoParamCandidate, total: &mut usize) -> bool {
         let index_key = IndexKey::ParamDeclaration { name: candidate.name.clone() };
         if self.index.contains_key(&index_key) {
-                continue;
+            return false;
         }
         if let Ok(re) = Regex::new(&candidate.message_match_regex) {
             self.param_regex_cache.insert(candidate.name.clone(), re);
@@ -718,9 +714,8 @@ impl EngineSnapshot {
                 log_stream_gated: false,
             }),
         );
-            new_this_line += 1;
+        *total += 1;
-            total += 1;
+        true
-        }
     }
 
     pub(crate) fn derive_log_stream_params_from_row(