fix: histogram interval (#4417)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit


- **New Features**
- Introduced a `histogram_interval` field to enhance query response
capabilities, allowing for more detailed data representation.
- Enhanced caching logic to set the `histogram_interval` based on the
`discard_interval`, improving the handling of cached query responses.

- **Improvements**
- Added conditional logic in response handling to include
`histogram_interval` when specific criteria are met, improving response
accuracy and robustness.
- Refined checks for empty responses, enhancing the robustness of the
merging process for query responses.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/common/meta/search.rs

@@ -58,6 +58,7 @@ pub struct MultiCachedQueryResponse {
     pub is_descending: bool,
     pub limit: i64,
     pub took: usize,
+    pub histogram_interval: i64,
 }
 
 #[derive(Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]

src/service/search/cache/cacher.rs

@@ -143,6 +143,9 @@ pub async fn check_cache(
         return MultiCachedQueryResponse::default();
     }
     let mut multi_resp = MultiCachedQueryResponse::default();
+    if discard_interval > -1 {
+        multi_resp.histogram_interval = discard_interval / 1000 / 1000;
+    }
     if get_config().common.use_multi_result_cache {
         let mut multi_res = crate::service::search::cluster::cache_multi::get_cached_results(
             query_key.to_owned(),

src/service/search/cache/mod.rs

@@ -256,6 +256,9 @@ pub async fn search(
     res.set_trace_id(trace_id.to_string());
     res.set_local_took(start.elapsed().as_millis() as usize, ext_took_wait);
 
+    if is_aggregate && c_resp.histogram_interval > -1 {
+        res.histogram_interval = Some(c_resp.histogram_interval);
+    }
     let req_stats = RequestStats {
         records: res.hits.len() as i64,
         response_time: time,
@@ -353,14 +356,19 @@ fn merge_response(
     };
 
     if cache_response.hits.is_empty()
-        && search_response.is_empty()
-        && search_response.first().unwrap().hits.is_empty()
-        && search_response.last().unwrap().hits.is_empty()
+        && !search_response.is_empty()
+        && search_response
+            .first()
+            .map_or(true, |res| res.hits.is_empty())
+        && search_response
+            .last()
+            .map_or(true, |res| res.hits.is_empty())
     {
         for res in search_response {
             cache_response.total += res.total;
             cache_response.scan_size += res.scan_size;
             cache_response.took += res.took;
+            cache_response.histogram_interval = res.histogram_interval;
         }
         return cache_response;
     }