Add support to retrieve logs from and endpoint based on the target instead of the id

[druanoor]

It will be very helpfull to be able to retreive logs from a probe execution based on the target instead of the id.

This will allow to add a link directly to the logs on the alerts. The id is not exported as a metric (and it shouldn't be) but the target is.

I understand that executions are not kept forever and may leed to inconsistences, but overall, being able to see the Probe logs next to the alerts will be a huge improvement

Something like:

http://blackbox-url/logs?target=prometheus.io