Feature request - Make Timeout configurable via a flag

[ahurtaud]

Hello,

In our setup of collectd-exporter, we would like to configure the timeout interval here:
https://github.com/prometheus/collectd_exporter/blob/master/main.go#L41-L44

The reason we want to do that is because, we have some metrics that are pushed to collectd every 5 iterations (because they are quite heavy to compute).
And our prometheus server is scraping at every iterations --- Marking stale the timeserie in every "gap" when removed by the exporter.

Before wrinting any code/sending any PR, I wanted to get approval by maintainers that this change might be accepted.

Thank you