X-Prometheus-Scrape-Timeout-Seconds and network latency

[ThomasSevestre]

By default ssl_exporter uses X-Prometheus-Scrape-Timeout-Seconds
When the timeout is triggered, the scrape timeout is also triggered and the job fails in prometheus.

Blackbox exporter uses a small offset to avoid this issue :
It is defined here : https://github.com/prometheus/blackbox_exporter/blob/master/main.go#L54
and used here :https://github.com/prometheus/blackbox_exporter/blob/master/prober/handler.go#L226

It would be a nice addition to this exporter