[FLINK-35886][task] Define RelativeClock interface

flink-core/src/main/java/org/apache/flink/util/clock/Clock.java

@@ -37,7 +37,7 @@
  * the system clock, so it never jumps. Relative time behaves similar to {@link System#nanoTime()}.
  */
 @PublicEvolving
-public abstract class Clock {
+public abstract class Clock implements RelativeClock {
 
     /** Gets the current absolute time, in milliseconds. */
     public abstract long absoluteTimeMillis();

flink-core/src/main/java/org/apache/flink/util/clock/RelativeClock.java

@@ -0,0 +1,35 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.util.clock;
+
+import org.apache.flink.annotation.PublicEvolving;
+
+/**
+ * A clock that gives access to relative time, similar to System#nanoTime(), however the progress of
+ * the relative time doesn't have to reflect the progress of a wall clock. Concrete classes can
+ * specify a different contract in that regard. Returned time must be monotonically increasing.
+ */
+@PublicEvolving
+public interface RelativeClock {
+    /** Gets the current relative time, in milliseconds. */
+    long relativeTimeMillis();
+
+    /** Gets the current relative time, in nanoseconds. */
+    long relativeTimeNanos();
+}