ddtrace/tracer: do not connect to trace-agent if tracing is disabled

[stephledev]

Version of dd-trace-go v1.52.0

Describe what happened: I'm setting the DD_TRACE_ENABLED environment variable to false to deactivate any tracing behaviour for an environment where the Datadog agent is not available. However during the call of tracer.Start() a new config will be created which as part of it will query the /info endpoint to query the trace agents capabilities (https://github.com/DataDog/dd-trace-go/blob/main/ddtrace/tracer/option.go#L331). This will end-up in an error as the function does not have a check if tracing is enabled or not:

Datadog Tracer v1.52.0 ERROR: Loading features: Get "http://localhost:8126/info": dial tcp [::1]:8126: connect: connection refused

Describe what you expected: The endpoint should not be queried if DD_TRACE_ENABLED is set to false.

Steps to reproduce the issue:

• Set DD_TRACE_ENABLED environment variable to false
• Start a minimal app

  func main() {
    tracer.Start()
    defer tracer.Stop()
  }
  

• Wait for the error to be raised

[knusbaum]

I have confirmed this is a bug. To be clear, this should not negatively affect your application after startup, but it is indeed a bug.