Add overhead metrics to info and memory stats. (#2674)

The corresponding doc PR of redis/redis#12913
---------

Co-authored-by: Oran Agra <oran@redislabs.com>

Author: CharlesChen888

commands/info.md

@@ -150,6 +150,7 @@ Here is the meaning of all fields in the **memory** section:
 *   `mem_replication_backlog`: Memory used by replication backlog
 *   `mem_total_replication_buffers`: Total memory consumed for replication buffers - Added in Redis 7.0.
 *   `mem_allocator`: Memory allocator, chosen at compile time.
+*   `mem_overhead_db_hashtable_rehashing`: Temporary memory overhead of database dictionaries currently being rehashed - Added in 8.0.
 *   `active_defrag_running`: When `activedefrag` is enabled, this indicates whether defragmentation is currently active, and the CPU percentage it intends to utilize.
 *   `lazyfree_pending_objects`: The number of objects waiting to be freed (as a
      result of calling `UNLINK`, or `FLUSHDB` and `FLUSHALL` with the **ASYNC**

commands/memory-stats.md

@@ -23,10 +23,13 @@ values. The following metrics are reported:
 *   `dbXXX`: For each of the server's databases, the overheads of the main and
      expiry dictionaries (`overhead.hashtable.main` and
     `overhead.hashtable.expires`, respectively) are reported in bytes
+*   `overhead.db.hashtable.lut`: Total overhead of dictionary buckets in databases (Added in Redis 8.0)
+*   `overhead.db.hashtable.rehashing`: Temporary memory overhead of database dictionaries currently being rehashed (Added in Redis 8.0) 
 *   `overhead.total`: The sum of all overheads, i.e. `startup.allocated`,
      `replication.backlog`, `clients.slaves`, `clients.normal`, `aof.buffer` and
      those of the internal data structures that are used in managing the
      Redis keyspace (see `INFO`'s `used_memory_overhead`)
+*   `db.dict.rehashing.count`: Number of DB dictionaries currently being rehashed (Added in Redis 8.0)
 *   `keys.count`: The total number of keys stored across all databases in the
      server
 *   `keys.bytes-per-key`: The ratio between `dataset.bytes` and `keys.count`