default void preWALAppend(ObserverContext<? extends RegionCoprocessorEnvironment> ctx, WALKey key,
    WALEdit edit) throws IOException {
  }
Called just before the WAL Entry is appended to the WAL. Implementing this hook allows
coprocessors to add extended attributes to the WALKey that then get persisted to the WAL, and
are available to replication endpoints to use in processing WAL Entries.
@param ctx the environment provided by the region server
@param key the WALKey associated with a particular append to a WAL
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/RegionObserver.java