Added default configuration for DataDog APM Tracer

[aantono]

Fixed DataDog APM initialization error due to missing default configs.

[mmatur]

LGTM

[mmatur]

Should be

if gc.Tracing.Zipkin != nil {
	log.Warn("Zipkin configuration will be ignored")
	gc.Tracing.Zipkin = nil
}
if gc.Tracing.Jaeger != nil {
	log.Warn("Jaeger configuration will be ignored")
	gc.Tracing.Jaeger = nil
}

or

log.Warn("Zipkin & Jaeger configuration will be ignored")
gc.Tracing.Jaeger = nil
gc.Tracing.Zipkin = nil

According to the chosen solution, you also need to update the Zipkin and Jaeger case

[mmatur]

I am not very a fan to have multiple conditional check for each tracer. I would propose different solution.

Solution 1

case jaeger.Name:
	if gc.Tracing.Jaeger == nil {
		gc.Tracing.Jaeger = &jaeger.Config{
			SamplingServerURL:  "http://localhost:5778/sampling",
			SamplingType:       "const",
			SamplingParam:      1.0,
			LocalAgentHostPort: "127.0.0.1:6831",
		}
	}
	if gc.Tracing.Zipkin != nil || gc.Tracing.DataDog != nil {
		log.Warn("Zipkin & DataDog configuration will be ignored")
		gc.Tracing.Zipkin = nil
                gc.Tracing.DataDog = nil
	}

Solution 2

	log.Info("Using Jaeger configuration other tracing configurations will be ignored")
	if gc.Tracing.Jaeger == nil {
		gc.Tracing.Jaeger = &jaeger.Config{
			SamplingServerURL:  "http://localhost:5778/sampling",
			SamplingType:       "const",
			SamplingParam:      1.0,
			LocalAgentHostPort: "127.0.0.1:6831",
		}
	}
	gc.Tracing.Zipkin = nil
	gc.Tracing.DataDog = nil

Personally I prefer the second solution.

WDYT?

[juliens]

LGTM

[emilevauge]

Thanks for your contribution @aantono, great job :)
But I tend to agree with @mmatur, this part is hard to maintain, his 2nd proposal seems great. We can merge this one and work on a refactor in another PR.
LGTM

[mmatur]

LGTM :)