[opentracing] add Compatibility Layer

[palazzem]

Overview

Compatibility layer for OpenTracing specification. The PR implements:

• a OT-compatible Tracer
• a Configuration struct to configure a Datadog Tracer
• the Span and SpanContext implementation
• the compatibility layer for OpenTracing Tags

This PR is an initial work to support entirely OpenTracing. In the current state, the OT-compatible Tracer wraps our previous implementation, so internals are the same. To set the Datadog Tracer as a GlobalTracer, you should add the following code to initialize the Tracer:

import (
    // datadog namespace is suggested
    datadog "github.com/DataDog/dd-trace-go/opentracing"
    opentracing "github.com/opentracing/opentracing-go"
)

func Example_initialization() {
    // create a Tracer configuration
    config := datadog.NewConfiguration()
    config.ServiceName = "api-intake"
    config.AgentHostname = "ddagent.consul.local"

    // initialize a Tracer and ensure a graceful shutdown
    // using the `closer.Close()`
    tracer, closer, err := datadog.NewTracer(config)
    if err != nil {
        // handle the configuration error
    }
    defer closer.Close()

    // set the Datadog tracer as a GlobalTracer
    opentracing.SetGlobalTracer(tracer)
}

Default values for Configuration are provided, and if you don't set a ServiceName, the binary name is used.

[masci]

just a couple of nits

[masci]

Should this live in tracer.go?

[masci]

config.SampleRate is missing

[masci]

This basically does all that TestConfiguration seems to do, let's merge the two?

[masci]

any reason why this is the only place where timezone is explicitly set to UTC?