Use an interface instead of a concrete type (#42)
Use an interface instead of a concrete type (#42)

* Use an interface instead of a concrete type
* Create observer interface until Prom 0.9.0 is out

Signed-off-by: Pavel Nikolov <c2644f216ff10b3eaa0a6a1b85bf84e3022b182d@fairfaxmedia.com.au>

uber/jaeger-lib,uber/jaeger-lib,uber/jaeger-lib