feat: add support for scheduling jobs using cron syntax

[FUSAKLA]

Instead of https://github.com/justwatchcom/sql_exporter/pull/30 implementing the CRON scheduling as suggested by @dewey

Not sure about the configuration if the precedence of the cron_schedule is ok or should bail out if both is configured?

PTAL

[FUSAKLA]

Possibly @dominikschulz :eyes:

[dewey]

Thanks for the PR. I'm going to take a look early next week :)

[FUSAKLA]

Great, thanks!

not sure about the vendoring issue in CI, locally go build -v ./... basses with no issues and the cron library is vendored :thinking:

[FUSAKLA]

@dewey friendly bump in case you may have forgotten?

[dewey]

Hey @FUSAKLA, not forgotten, just busy :)

I just tried it locally and I'm getting the same error as CI. The robfig cron library is not vendored that's why it doesn't work. After running go mod vendor it builds fine. Can you run a go mod vendor?

sql_exporter|fus-cronjob ⇒ go build -v ./...
config.go:16:2: cannot find package "." in:
    /Users/philippdefner/mod/github.com/justwatchcom/sql_exporter/vendor/github.com/robfig/cron/v3

[FUSAKLA]

:facepalm: I had a vendor ignored by the global git config. Sorry my bad. Should be fixed now.

[dewey]

Looks good now @FUSAKLA. Thanks for the feature!

[FUSAKLA]

Thanks for accepting it! :)