feat(unified): add index scoring model config (#116210)

* feat(unified): add bm25 index scoring model

We want try BM25 scoring model since they have global scoring which we can probably re-use for fan-in/fan-out logic

https://github.com/blevesearch/bleve/blob/32d98823c4b7482c62cc6c847508ed7659c23c37/docs/scoring.md#global-scoring

* fix(plugins): update plugin test data

pkg/setting/setting.go

@@ -600,6 +600,7 @@ type Cfg struct {
 	IndexRebuildInterval                       time.Duration
 	IndexCacheTTL                              time.Duration
 	IndexMinUpdateInterval                     time.Duration // Don't update index if it was updated less than this interval ago.
+	IndexScoringModel                          string        // Note: Temporary config to switch the index scoring model and will be removed soon.
 	MaxFileIndexAge                            time.Duration // Max age of file-based indexes. Index older than this will be rebuilt asynchronously.
 	MinFileIndexBuildVersion                   string        // Minimum version of Grafana that built the file-based index. If index was built with older Grafana, it will be rebuilt asynchronously.
 	EnableSharding                             bool

pkg/setting/setting_unified_storage.go

@@ -123,6 +123,10 @@ func (cfg *Cfg) setUnifiedStorageConfig() {
 	cfg.IndexRebuildInterval = section.Key("index_rebuild_interval").MustDuration(24 * time.Hour)
 	cfg.IndexCacheTTL = section.Key("index_cache_ttl").MustDuration(10 * time.Minute)
 	cfg.IndexMinUpdateInterval = section.Key("index_min_update_interval").MustDuration(0)
+	cfg.IndexScoringModel = section.Key("index_scoring_model").MustString("")
+	if cfg.IndexScoringModel != "" {
+		cfg.Logger.Info("Index scoring model set", "model", cfg.IndexScoringModel)
+	}
 	cfg.SprinklesApiServer = section.Key("sprinkles_api_server").String()
 	cfg.SprinklesApiServerPageLimit = section.Key("sprinkles_api_server_page_limit").MustInt(10000)
 	cfg.CACertPath = section.Key("ca_cert_path").String()