Unified Storage Indexer: Add back metrics (#97310)

* Adds back indexer metrics. Uses config values instead of hardcoded ones.

* cast to int64

* remove unused func

* Index metrics impl doesn't depend on Bleve. Adds a TotalDocs func to SearchBackend interface.

* adds config setting for index_min_count

* rename arg

* rename metric label to namespace instead of slug

* adds default "do nothing" case to satisfy linter

* moves bleve index metrics to search package

* make bleve backend private, dont need to pass in prom reg

* imports

* adds bleve metrics to resource package to avoid circular deps

pkg/setting/setting.go

@@ -527,11 +527,12 @@ type Cfg struct {
 	ShortLinkExpiration int
 
 	// Unified Storage
-	UnifiedStorage    map[string]UnifiedStorageConfig
-	IndexPath         string
-	IndexWorkers      int
-	IndexMaxBatchSize int
-	IndexListLimit    int
+	UnifiedStorage     map[string]UnifiedStorageConfig
+	IndexPath          string
+	IndexWorkers       int
+	IndexMaxBatchSize  int
+	IndexFileThreshold int
+	IndexMinCount      int
 }
 
 type UnifiedStorageConfig struct {