Influx backup command support backup config of influx without backup data

[b7wch]

Proposal:
Influx backup command support backup config of influx without backup data, or support --start param like v1.8 to permit user to determine records from which date to backup.

Current behavior:
Now influx backup will backup all data and config by default , and couldn't choice backup config only.

Desired behavior:
Support choice backup config only without data.

Alternatives considered:
influx backup support -start param like v1.8

Use case:
Sometimes data in influxdb is huge, but user just take care about config.