add missing RocksDbCardStore test

core/src/test/scala/filodb.core/memstore/ratelimit/RocksDbCardinalityStoreSpec.scala

@@ -0,0 +1,29 @@
+package filodb.core.memstore.ratelimit
+
+import org.scalatest.funspec.AnyFunSpec
+import org.scalatest.matchers.should.Matchers
+
+import filodb.core.memstore.ratelimit.CardinalityStore._
+import filodb.core.MetricsTestData
+
+class RocksDbCardinalityStoreSpec extends AnyFunSpec with Matchers {
+  it ("should correctly return overflow record") {
+    val dset = MetricsTestData.timeseriesDatasetMultipleShardKeys
+    val shard = 0
+    val numOverflow = 123
+
+    val db = new RocksDbCardinalityStore(dset.ref, shard)
+
+    (0 until MAX_RESULT_SIZE + numOverflow).foreach{ i =>
+      val prefix = Seq("ws", "ns", s"metric-$i")
+      db.store(CardinalityRecord(shard, prefix, 1, 1, 1, 1))
+    }
+
+    Seq(Nil, Seq("ws"), Seq("ws", "ns")).foreach{ prefix =>
+      val res = db.scanChildren(prefix, 3)
+      res.size shouldEqual MAX_RESULT_SIZE + 1  // one extra for the overflow CardinalityRecord
+      res.contains(CardinalityRecord(shard, OVERFLOW_PREFIX,
+        numOverflow, numOverflow, numOverflow, numOverflow)) shouldEqual true
+    }
+  }
+}