Add trait StreamInstrument

[cjwcommuny]

Feature Request

Motivation

For Future, tracing has the Instrument trait. But for Stream, tracing doesn't have similar trait to use.

Proposal

Add StreamInstrument trait similar to the current Instrument trait.

[mladedav]

This is already implemented, but tracing does not depend on futures so you'll have to use tracing-futures with the futures-03 feature.

Then it should work out of the box the same way as with Future, you just call instrument on your Stream and then use the resulting object as if it was the original.