Option to add a unique id in URLs of same probe request

[agirbal]

Context: each probe path may hit independent systems that are independent, with their own set of processing, caching, logging. Need: 1a. while the results of all probes for the same run are gathered and logged by the client, it would be useful to be able to easily find a specific run in a sub-system log, for example to understand why a run was slow. 1b. allow easy joins in later data processing, to gather many internal stats for the run. 2. some path may have caching layers while others don't, and having a unique ID could ensure caching is not part of the equation (while in some other cases caching may be interesting to measure).