Use Prometheus format for durations

[santihernandezc]

Our Receivers API is using the Go format for the lastAttemptDuration field. This goes against what Prometheus uses for other durations.
Excerpt from the docs:

<duration>: a duration matching the regular expression ((([0-9]+)y)?(([0-9]+)w)?(([0-9]+)d)?(([0-9]+)h)?(([0-9]+)m)?(([0-9]+)s)?(([0-9]+)ms)?|0), e.g. 1d, 1h30m, 5m, 10s