add support for trace.SpanContextFromContext so it is possible to get a tracer and span info to injected in logs

[hagen-p]

I want to inject Trace information in my logs ,

To this I added the following code to my application, with this code I do not see the trace information printed as the SpanCtx is not valid.

I ideally I would like to call trace.SpanContextFromContext. to get the Span Context for the (global) Tracer

And use that to retrieve the current span/trace information

This might also be useful for adding span attributes

 package main

import (
	"github.com/sirupsen/logrus"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/trace"
)
func main() {
	ctx := context.TODO()
	ctx, span := otel.Tracer("github.com/my/repo").Start(ctx, "log info")
	defer span.End()
	spanCtx := trace.SpanContextFromContext(ctx)
	logrus.WithFields(LogrusFields(spanCtx)).Info("loaded users")
}

func LogrusFields(spanCtx trace.SpanContext) logrus.Fields {
	if !spanCtx.IsValid() {
		return logrus.Fields{}
	}
	return logrus.Fields{
		"span_id":     spanCtx.SpanID().String(),
		"trace_id":    spanCtx.TraceID().String(),
		"trace_flags": spanCtx.TraceFlags().String(),
	}
}