Allow adding custom conditions to `Instrumenter#shouldStart()`

[mateuszrzeszutek]

Is your feature request related to a problem? Please describe.
Currently there is no way to embed some additional conditions into the Instrumenter#shouldStart() method. There are several instrumentations that have additional checks (netty-4.1 client, jaxrs-1.1, cxf, ...)

Describe the solution you'd like
We could add an additional interface that accepts (REQUEST, Context) and returns a boolean - and add a new configuration knob for it to the InstrumenterBuilder.

Describe alternatives you've considered
Leave the code in those instrumentations as it is.

Additional context
#4381 (comment)

[trask]

Closing, I think this has been addressed by span suppression mechanism.