Add trace options

[a-roberts]

I'm testing appmetrics-zipkin with new versions of IBM Cloud Private regularly, I have a sample that uses it but will often silently not be working as expected.

It'd be useful to have environment variables we can set that, when set, enables extra output to be printed (we'll essentially be tracing the tracer).

Here's what I'm proposing.

ZIPKIN_TRACE_PRINT_SPANS
If set, print the span information that we're creating on an endpoint being used

ZIPKIN_TRACE_PRINT_TRACES
If set, print the trace information (linking multiple spans between multiple services for example).

ZIPKIN_TRACE_PRINT_SERVER_IP
If set, print the Zipkin server IP we're trying to send data to.

ZIPKIN_TRACE_PRINT_SERVER_PORT
If set, print the Zipkin server port we're trying to send data to.

ZIPKIN_TRACE_CLIENT_VERSION
If set, print the Zipkin client module version that we're using.

ZIPKIN_TRACE_APPMETRICS_VERSION
If set, print the version of appmetrics we're using (as we're piggybacking off appmetrics probes AFAIK, this is important to know).

ZIPKIN_TRACE_SERVER_VERSION
If set, print the version of Zipkin running on the server (not sure how backwards compatible Zipkin servers are with clients so this may also be useful to know).