fix dynamic flags

[MegaByte875]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

Description:

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

[MegaByte875]

creating cluster and updating config:

    config:
      logtostder: "true"
      redirect_stdout: "false"
      rocksdb_db_options: '{"max_subcompactions":"3","max_background_jobs":"3"}'
      stderrthreshold: "0"

sh-4.2#  cat data/storage/nebula/0/0/data/LOG |grep max_background_jobs
2023/10/04-10:16:47.234764 140250029536064             Options.max_background_jobs: 3
sh-4.2#  cat data/storage/nebula/1/*/data/LOG |grep max_background_jobs
2023/10/04-10:16:47.185186 140250029536064             Options.max_background_jobs: 3
2023/10/04-10:16:47.170256 140250029536064             Options.max_background_jobs: 3
2023/10/04-10:16:47.162272 140250029536064             Options.max_background_jobs: 3
2023/10/04-10:16:47.152780 140250029536064             Options.max_background_jobs: 3
2023/10/04-10:16:47.177669 140250029536064             Options.max_background_jobs: 3

$ curl -s 10.244.1.189:19779/flags |grep rocksdb_db_options
rocksdb_db_options="{"max_subcompactions":"3","max_background_jobs":"3"}"

[veezhang]

lgtm