chore: minor refactor on etcd kvbackend #16157

Job	Run time
Check typos and docs	7s
Clippy	5m 21s
coverage	15m 49s
Check (windows-2022)	5m 1s
Build GreptimeDB binaries (ubuntu-20.04)	8m 59s
Check (ubuntu-20.04)	1m 50s
Rustfmt	27s
Check License Header	1m 36s
Toml Check	28s
Unstable Fuzz Test (unstable_fuzz_create_table_standalone)	14m 45s
Fuzz Test (fuzz_create_table)	5m 6s
Sqlness Test (ubuntu-20.04)	2m 22s
Fuzz Test (fuzz_alter_table)	6m 40s
Sqlness Test with Kafka Wal (ubuntu-20.04)	5m 32s
Fuzz Test (fuzz_create_database)	6m 6s
Fuzz Test (fuzz_create_logical_table)	8m 44s
Fuzz Test (fuzz_alter_logical_table)	5m 11s
Fuzz Test (fuzz_insert)	8m 51s
Fuzz Test (fuzz_insert_logical_table)	5m 14s
	1h 48m 9s

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