Clarify --interval option in external Exporters

Hi,

I notice this is not very clear - It seems to be expecting the unit
"seconds", "minutes" etc however it is not clear how to specify it. Help
message does not provide any examples

[root@ip-172-31-16-160 ~]# pmm-admin add external:service processes
--service-port=9256 --interval 10
Error: invalid argument "10" for "--interval" flag: time: missing unit in
duration 10
Usage:
pmm-admin add external:service job_name [instance] --service-port=port
[flags]

Flags:
--force skip reachability check, overwrite scrape job
parameters
-h, --help help for external:service
--interval duration scrape interval
--path string metrics path
--scheme string protocol scheme for scrapes
--timeout duration scrape timeout

Global Flags:
-c, --config-file string PMM config file (default
"/usr/local/percona/pmm-client/pmm.yml")
--service-port int service port
--verbose verbose output

invalid argument "10" for "--interval" flag: time: missing unit in duration
10

–
Peter Zaitsev, CEO, Percona

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