Add commandline option to disable schedule-related metrics

[wolfmd]

Schedule metrics can be quite verbose for larger pagerduty accounts. Add an option to disable the pagerduty_schedule_* metrics at runtime.

This will allow users to cut down on cardinality and metrics that aren't needed for their usecase.

For an example in my particular environment
Without the disable-schedule flag set

$ curl localhost:8080/metrics | wc -l
    7477

With the disable-schedule flag set

$ curl localhost:8080/metrics | wc -l
     720

[sonarqubecloud]

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[mblaschke]

i would like to merge the flag --scrape.time.schedule= which then is based by default on --scrape.time= if not set.
So if you set --scrape.time.schedule= to 0 it will disable it.

This could be done for other scrapes as well

[wolfmd]

If you've got a PR for that ready I'd be happy to decline this in favor of whatever you feel makes sense

[mblaschke]

please try webdevops/pagerduty-exporter:main with env var SCRAPE_TIME_SCHEDULE=0