Summary: Observer API for OpenTracing-Go Tracers

Description:

OTObserver can be used to watch the span events like StartSpan(),
SetOperationName(), SetTag() and Finish(). A need for observers arose when
information (metrics) more than just the latency information was required for
the spans, in the distributed tracers. But, there can be a lot of metrics in
different domains and adding such metrics to any one (client) tracer breaks
cross-platform compatibility. There are various ways to avoid such issues,
however, an observer pattern is cleaner and provides loose coupling between the
packages exporting metrics (on span events) and the tracer.

This information can be in the form of hardware metrics, RPC metrics, useful
metrics exported out of the kernel or other metrics, profiled for a span. These
additional metrics can help us in getting better Root-cause analysis. With that
being said, its not just for calculation of metrics, it can be used for anything
which needs watching the span events.

Maintainer: pterjan