view cluster no-op or importer mode

[wentaojin]

Feature Request

Is your feature request related to a problem? Please describe:

After using tidb-lightning, can not confirm tidb cluster current mode is no-op or importer mode

Describe the feature you'd like:

tidb-lightning is missing command to view cluster no-op or importer mode

[kennytm]

The "import mode" setting is tracked directly by each TiKV node, and the current value isn't exposed by any API yet. There is currently a PR for TiKV 3.0 tikv/tikv#4206 which will eventually expose such metrics to Prometheus. After that PR is merged, we could use the metrics to distinguish between the two modes.

Metric label (tikv_config_rocksdb{name="…"})	Normal mode	Import mode
level0_stop_writes_trigger	small (36)	large (230)
level0_slowdown_writes_trigger	small (20)	large (230)
soft_pending_compaction_bytes_limit	large (64 GiB)	0
hard_pending_compaction_bytes_limit	large (256 GiB)	0