Unistore: Reuse MySQL and Postgres Grafana core config instead of the object (#94223)

* Reuse MySQL and Postgres Grafana config instead of the object

- Only reuse the Grafana DB object for SQLite. Support for SQLite will be added in a different PR
- Fail when reusing the Grafana DB object if it is using DB instrumentation
- In the case that we have to reuse a Grafana DB with its instrumentation, fail with an error that describes a workaround
- Add regression tests to reproduce incident 2144

* remove temp file

* fix linter

* fix linter x2

* fix linter x3

pkg/storage/unified/sql/test/integration_test.go

@@ -31,9 +31,8 @@ func newServer(t *testing.T) (sql.Backend, resource.ResourceServer) {
 
 	dbstore := infraDB.InitTestDB(t)
 	cfg := setting.NewCfg()
-	features := featuremgmt.WithFeatures()
 
-	eDB, err := dbimpl.ProvideResourceDB(dbstore, cfg, features, nil)
+	eDB, err := dbimpl.ProvideResourceDB(dbstore, cfg, nil)
 	require.NoError(t, err)
 	require.NotNil(t, eDB)