Support Datadog metrics over unix socket

[lhriley]

According to the documentation, the datadog opentracing plugin supports a variety of transport options not currently supported by this framework.

Specifically, it looks like we should be checking for the config in here:

nginx-opentracing/example/trivial/datadog/go/server.go

Lines 19 to 20 in c2b969f

	var collectorHost = flag.String("collector_host", "dd-agent", "Host for Datadog agent")
	var collectorPort = flag.String("collector_port", "8126", "Port for Datadog agent")

From their configuration docs:

If the Agent URL is specified, then it overrides the Agent host and Agent port settings.

References:
DataDog/dd-opentracing-cpp#125
https://github.com/DataDog/dd-opentracing-cpp/blob/master/doc/configuration.md#agent-url