Request- or context-rooted measurements

[bassosimone]

As a user of the netx API, I would like to be able to automatically know what events have been generated by a specific *http.Request. In the same vein, I would like to know what events have been generated by code using a specific context.Context. Because we can assign a context to a request, the most general case is the latter. That is, we could always implement request-rooted measurements using context-rooted measurements.

The use case for this functionality is easily getting measurements that are part of the same round-trip, or round-trips, without the need to join them in post-processing and/or without preventing concurrent round-trips, like we do in ooni/probe-engine now.

With this functionality implemented, I doubt we will ever need to join measurements, because we could know by definition that a specific request is the root cause of a specific list of low-level measurements.

This is not super urgent, because we can thrive in ooni/probe-engine even without this functionality, but our life would be easier with this functionality merged. Also, this probably solves the most pressing concern of netx's original design, i.e. joining measurements.

[bassosimone]

This has now landed in master