Polish instrument initialization and lookups

[ktoso]

related question is how the InstrumentationSystem should be configured,
for instance should there be only one instrument, one of a kind and so on:

    // FIXME: smarter impl
    public static var tracer: TracingInstrument {
        self.lock.withReaderLock {
            let tracer: TracingInstrument? = self._tracer
            let res: TracingInstrument = tracer ?? NoOpTracingInstrument()
            return res
        }
    }

BTW the code above will not find a tracer if its provided by official and public MultiplexInstrument

Originally posted by @pokryfka in #31 (comment)

[ktoso]

Yeah the initialization and obtaining of tracers is not polished yet, can we not conflate the topics though?
New ticket for the fixing of initialization and tracer lookup would be useful.

I agree that it's not clear how we should deal with multiples if there's a tracer and non tracers yet; though that should be solvable IMO.

[slashmo]

@ktoso Can this be closed now as we already merged #101?

[ktoso]

Good point, I think so 👍