feat(observability): add index cache hit/miss and eviction metrics

surfsense_backend/app/observability/metrics.py

@@ -305,6 +305,22 @@ def _etl_cache_evictions():
     )
 
 
+@lru_cache(maxsize=1)
+def _index_cache_lookups():
+    return _get_meter().create_counter(
+        "surfsense.index.cache.lookups",
+        description="Count of index (chunk+embedding) cache lookups by outcome (hit/miss).",
+    )
+
+
+@lru_cache(maxsize=1)
+def _index_cache_evictions():
+    return _get_meter().create_counter(
+        "surfsense.index.cache.evictions",
+        description="Count of index cache entries evicted, by phase.",
+    )
+
+
 @lru_cache(maxsize=1)
 def _celery_heartbeat_refreshes():
     return _get_meter().create_counter(
@@ -708,6 +724,28 @@ def record_etl_cache_eviction(count: int, *, phase: str) -> None:
     _add(_etl_cache_evictions(), count, {"phase": phase})
 
 
+def record_index_cache_lookup(
+    *, embedding_model: str | None, chunker_kind: str | None, outcome: str
+) -> None:
+    """Record an index-cache lookup. ``outcome`` is ``hit`` or ``miss``."""
+    _add(
+        _index_cache_lookups(),
+        1,
+        {
+            "embedding.model": embedding_model or "unknown",
+            "chunker.kind": chunker_kind or "unknown",
+            "outcome": outcome,
+        },
+    )
+
+
+def record_index_cache_eviction(count: int, *, phase: str) -> None:
+    """Record evicted entries. ``phase`` is ``ttl`` or ``size``."""
+    if count <= 0:
+        return
+    _add(_index_cache_evictions(), count, {"phase": phase})
+
+
 def record_celery_heartbeat_refresh(*, heartbeat_type: str) -> None:
     _add(_celery_heartbeat_refreshes(), 1, {"heartbeat.type": heartbeat_type})
 
@@ -908,6 +946,8 @@ __all__ = [
     "record_etl_cache_lookup",
     "record_etl_extract_duration",
     "record_etl_extract_outcome",
+    "record_index_cache_eviction",
+    "record_index_cache_lookup",
     "record_indexing_document_duration",
     "record_indexing_document_outcome",
     "record_interrupt",