KAFKA-8715; Fix buggy reliance on state timestamp in static member.id generation (#7116)

The bug is that we accidentally used the current state timestamp for the group instead of the real current time. When a group is first loaded, this timestamp is not initialized, so this resulted in a `NoSuchElementException`. Additionally this violated the intended uniqueness of the memberId, which could have broken the group instance fencing. Fix is made and unit test to make sure the timestamp is properly encoded within the returned member.id.

Reviewers: Ismael Juma <ismael@juma.me.uk>, Guozhang Wang <wangguoz@gmail.com>, Jason Gustafson <jason@confluent.io>

core/src/main/scala/kafka/coordinator/group/GroupMetadata.scala

@@ -365,7 +365,7 @@ private[group] class GroupMetadata(val groupId: String, initialState: GroupState
       case None =>
         clientId + GroupMetadata.MemberIdDelimiter + UUID.randomUUID().toString
       case Some(instanceId) =>
-        instanceId + GroupMetadata.MemberIdDelimiter + currentStateTimestamp.get
+        instanceId + GroupMetadata.MemberIdDelimiter + UUID.randomUUID().toString
     }
   }
 

core/src/test/scala/unit/kafka/coordinator/group/GroupCoordinatorTest.scala

@@ -873,6 +873,23 @@ class GroupCoordinatorTest {
     assertEquals(Errors.FENCED_INSTANCE_ID, invalidHeartbeatResult)
   }
 
+  @Test
+  def shouldGetDifferentStaticMemberIdAfterEachRejoin(): Unit = {
+    val initialResult = staticMembersJoinAndRebalance(leaderInstanceId, followerInstanceId)
+
+    val timeAdvance = 1
+    var lastMemberId = initialResult.leaderId
+    for (_ <- 1 to 5) {
+      EasyMock.reset(replicaManager)
+
+      val joinGroupResult = staticJoinGroup(groupId, JoinGroupRequest.UNKNOWN_MEMBER_ID,
+        leaderInstanceId, protocolType, protocols, clockAdvance = timeAdvance)
+      assertTrue(joinGroupResult.memberId.startsWith(leaderInstanceId.get))
+      assertNotEquals(lastMemberId, joinGroupResult.memberId)
+      lastMemberId = joinGroupResult.memberId
+    }
+  }
+
   @Test
   def testOffsetCommitDeadGroup() {
     val memberId = "memberId"