Fix getting-started.md Run function (#2527)

* Fix getting-started.md Run function, it assigns this new context to a variable shared between connections in to accept loop. Thus creating a growing chain of contexts. so every calculate fibonacci request, all spans in a trace.

* add a comment explaining the reason for that new variable

* update example fib

example/fib/app.go

@@ -44,16 +44,16 @@ func NewApp(r io.Reader, l *log.Logger) *App {
 // Run starts polling users for Fibonacci number requests and writes results.
 func (a *App) Run(ctx context.Context) error {
 	for {
-		var span trace.Span
-		ctx, span = otel.Tracer(name).Start(ctx, "Run")
+		// Each execution of the run loop, we should get a new "root" span and context.
+		newCtx, span := otel.Tracer(name).Start(ctx, "Run")
 
-		n, err := a.Poll(ctx)
+		n, err := a.Poll(newCtx)
 		if err != nil {
 			span.End()
 			return err
 		}
 
-		a.Write(ctx, n)
+		a.Write(newCtx, n)
 		span.End()
 	}
 }

website_docs/getting-started.md

@@ -184,16 +184,16 @@ Start by instrumenting the `Run` method.
 // Run starts polling users for Fibonacci number requests and writes results.
 func (a *App) Run(ctx context.Context) error {
 	for {
-		var span trace.Span
-		ctx, span = otel.Tracer(name).Start(ctx, "Run")
+		// Each execution of the run loop, we should get a new "root" span and context.
+		newCtx, span := otel.Tracer(name).Start(ctx, "Run")
 
-		n, err := a.Poll(ctx)
+		n, err := a.Poll(newCtx)
 		if err != nil {
 			span.End()
 			return err
 		}
 
-		a.Write(ctx, n)
+		a.Write(newCtx, n)
 		span.End()
 	}
 }