fix(unified): err on timeout to open index (#115953)

* fix(unified): default index path to ephemeral storage mount path

Signed-off-by: Rafael Paulovic <rafael.paulovic@grafana.com>

* Revert "fix: use memory index if index file already open (#115720)"

This reverts commit dc4c106e91.

* fix(unified): set index_path for tests

* chore(unified): re-add bolt open timeout and test for error handling

* chore(unified): return err on timeout

* chore(unified): revert changes to default, use DataPath if index_path not set

* chore(unified): add defaults.ini entry for unified_storage

This is needed to override using env. vars

* chore: revert unrelated diff

* chore: address code review comments

- reduce bolt timeout to 1s
- remove errIndexLocked err type
- add more information about index_path in defaults.ini

---------

Signed-off-by: Rafael Paulovic <rafael.paulovic@grafana.com>

pkg/setting/setting.go

@@ -588,8 +588,10 @@ type Cfg struct {
 	// Unified Storage
 	UnifiedStorage map[string]UnifiedStorageConfig
 	// DisableDataMigrations will disable resources data migration to unified storage at startup
-	DisableDataMigrations                      bool
-	MaxPageSizeBytes                           int
+	DisableDataMigrations bool
+	MaxPageSizeBytes      int
+	// IndexPath the directory where index files are stored.
+	// Note: Bleve locks index files, so mounts cannot be shared between multiple instances.
 	IndexPath                                  string
 	IndexWorkers                               int
 	IndexRebuildWorkers                        int