[prometheus-to-sd] make reading `process_start_time_seconds` metric more configurable

the prometheus-to-sd exporter in translator.go always expects a unix epoch reported in a metric named process_start_time_seconds. I'm asking that this metric name be configurable, that we also accept a "uptime" metric in seconds rather then a unix epoch and also support a mode where the uptime is just assumed to be the same uptime as prometheus-to-sd 's uptime.

thanks