ref: reshard backend (#38203)

backend (0) was approaching a P90 of 19.8 minutes (timeout)

.github/workflows/backend.yml

@@ -68,12 +68,12 @@ jobs:
       fail-fast: false
       matrix:
         # XXX: When updating this, make sure you also update MATRIX_INSTANCE_TOTAL.
-        instance: [0, 1, 2]
+        instance: [0, 1, 2, 3]
         pg-version: ['9.6']
 
     env:
       # XXX: MATRIX_INSTANCE_TOTAL must be hardcoded to the length of strategy.matrix.instance.
-      MATRIX_INSTANCE_TOTAL: 3
+      MATRIX_INSTANCE_TOTAL: 4
       MIGRATIONS_TEST_MIGRATE: 1
 
     steps: