[Chore] Edit metadata cache supported values description (#6252)

Author: SungJin1212

docs/blocks-storage/querier.md

@@ -1020,7 +1020,8 @@ blocks_storage:
       [subrange_ttl: <duration> | default = 24h]
 
     metadata_cache:
-      # Backend for metadata cache, if not empty. Supported values: memcached.
+      # Backend for metadata cache, if not empty. Supported values: memcached,
+      # redis, and '' (disable).
       # CLI flag: -blocks-storage.bucket-store.metadata-cache.backend
       [backend: <string> | default = ""]
 

docs/blocks-storage/store-gateway.md

@@ -1135,7 +1135,8 @@ blocks_storage:
       [subrange_ttl: <duration> | default = 24h]
 
     metadata_cache:
-      # Backend for metadata cache, if not empty. Supported values: memcached.
+      # Backend for metadata cache, if not empty. Supported values: memcached,
+      # redis, and '' (disable).
       # CLI flag: -blocks-storage.bucket-store.metadata-cache.backend
       [backend: <string> | default = ""]
 

docs/configuration/config-file-reference.md

@@ -1568,7 +1568,8 @@ bucket_store:
     [subrange_ttl: <duration> | default = 24h]
 
   metadata_cache:
-    # Backend for metadata cache, if not empty. Supported values: memcached.
+    # Backend for metadata cache, if not empty. Supported values: memcached,
+    # redis, and '' (disable).
     # CLI flag: -blocks-storage.bucket-store.metadata-cache.backend
     [backend: <string> | default = ""]
 

pkg/storage/tsdb/caching_bucket.go

@@ -24,6 +24,8 @@ import (
 )
 
 var (
+	supportedMetadataCacheBackends = []string{CacheBackendMemcached, CacheBackendRedis}
+
 	errUnsupportedChunkCacheBackend = errors.New("unsupported chunk cache backend")
 )
 
@@ -140,7 +142,7 @@ type MetadataCacheConfig struct {
 }
 
 func (cfg *MetadataCacheConfig) RegisterFlagsWithPrefix(f *flag.FlagSet, prefix string) {
-	f.StringVar(&cfg.Backend, prefix+"backend", "", fmt.Sprintf("Backend for metadata cache, if not empty. Supported values: %s.", CacheBackendMemcached))
+	f.StringVar(&cfg.Backend, prefix+"backend", "", fmt.Sprintf("Backend for metadata cache, if not empty. Supported values: %s, and '' (disable).", strings.Join(supportedMetadataCacheBackends, ", ")))
 
 	cfg.Memcached.RegisterFlagsWithPrefix(f, prefix+"memcached.")
 	cfg.Redis.RegisterFlagsWithPrefix(f, prefix+"redis.")