Add latest_fork_time to INFO STATS

Sometimes we want to know the last fork time, this can help
user to analyze the stability since right now fork is still
a costly operation and user can monitor forks by drawing graphs
based on these fork fields.

Signed-off-by: Binbin <[email protected]>

src/server.c

@@ -2499,6 +2499,7 @@ void resetServerStats(void) {
     server.stat_fork_time = 0;
     server.stat_fork_rate = 0;
     server.stat_total_forks = 0;
+    server.stat_last_fork_time = 0;
     server.stat_rejected_conn = 0;
     server.stat_sync_full = 0;
     server.stat_sync_partial_ok = 0;
@@ -5710,6 +5711,7 @@ sds genValkeyInfoString(dict *section_dict, int all_sections, int everything) {
             "pubsub_patterns:%lu\r\n", dictSize(server.pubsub_patterns),
             "pubsubshard_channels:%llu\r\n", kvstoreSize(server.pubsubshard_channels),
             "latest_fork_usec:%lld\r\n", server.stat_fork_time,
+            "latest_fork_time:%lld\r\n", server.stat_last_fork_time,
             "total_forks:%lld\r\n", server.stat_total_forks,
             "migrate_cached_sockets:%ld\r\n", dictSize(server.migrate_cached_sockets),
             "slave_expires_tracked_keys:%zu\r\n", getReplicaKeyWithExpireCount(),
@@ -6304,6 +6306,7 @@ int serverFork(int purpose) {
         server.stat_fork_time = ustime() - start;
         server.stat_fork_rate =
             (double)zmalloc_used_memory() * 1000000 / server.stat_fork_time / (1024 * 1024 * 1024); /* GB per second. */
+        server.stat_last_fork_time = start;
         latencyAddSampleIfNeeded("fork", server.stat_fork_time / 1000);
 
         /* The child_pid and child_type are only for mutually exclusive children.

src/server.h

@@ -1805,6 +1805,7 @@ struct valkeyServer {
     long long stat_aof_rewrites;                   /* number of aof file rewrites performed */
     long long stat_aofrw_consecutive_failures;     /* The number of consecutive failures of aofrw */
     long long stat_rdb_saves;                      /* number of rdb saves performed */
+    long long stat_last_fork_time;                 /* Last time to start a successful fork(). */
     long long stat_fork_time;                      /* Time needed to perform latest fork() */
     double stat_fork_rate;                         /* Fork rate in GB/sec. */
     long long stat_total_forks;                    /* Total count of fork. */