swarm: integrate OpenTracing; propagate ctx to internal APIs (#17169)

* swarm: propagate ctx, enable opentracing

* swarm/tracing: log error when tracing is misconfigured

swarm/spancontext/spancontext.go

@@ -0,0 +1,49 @@
+package spancontext
+
+import (
+	"context"
+
+	opentracing "github.com/opentracing/opentracing-go"
+)
+
+func WithContext(ctx context.Context, sctx opentracing.SpanContext) context.Context {
+	return context.WithValue(ctx, "span_context", sctx)
+}
+
+func FromContext(ctx context.Context) opentracing.SpanContext {
+	sctx, ok := ctx.Value("span_context").(opentracing.SpanContext)
+	if ok {
+		return sctx
+	}
+
+	return nil
+}
+
+func StartSpan(ctx context.Context, name string) (context.Context, opentracing.Span) {
+	tracer := opentracing.GlobalTracer()
+
+	sctx := FromContext(ctx)
+
+	var sp opentracing.Span
+	if sctx != nil {
+		sp = tracer.StartSpan(
+			name,
+			opentracing.ChildOf(sctx))
+	} else {
+		sp = tracer.StartSpan(name)
+	}
+
+	nctx := context.WithValue(ctx, "span_context", sp.Context())
+
+	return nctx, sp
+}
+
+func StartSpanFrom(name string, sctx opentracing.SpanContext) opentracing.Span {
+	tracer := opentracing.GlobalTracer()
+
+	sp := tracer.StartSpan(
+		name,
+		opentracing.ChildOf(sctx))
+
+	return sp
+}