Merge reader and writer adapters into one

Code is a bit shorter this way

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

bridge/opentracing/bridge.go

@@ -620,23 +620,43 @@ func (s fakeSpan) SpanContext() trace.SpanContext {
 	return s.sc
 }
 
-// adapt OpenTracing TextMapWriter to OpenTelemetry propagation.TextMapCarrier
-type textMapWriterAdapter struct {
-	t ot.TextMapWriter
+// adapt OpenTracing TextMapReader or TextMapWriter to OpenTelemetry propagation.TextMapCarrier
+type textMapAdapter struct {
+	r ot.TextMapReader
+	w ot.TextMapWriter
 }
 
-func (a textMapWriterAdapter) Get(key string) string {
-	// not implemented
-	return ""
+func (a textMapAdapter) Get(key string) string {
+	if a.r == nil {
+		return "" // not implemented on writer
+	}
+	var ret string
+	_ = a.r.ForeachKey(func(k, v string) error {
+		if key == k {
+			ret = v
+		}
+		return nil
+	})
+	return ret
 }
 
-func (a textMapWriterAdapter) Set(key string, value string) {
-	a.t.Set(key, value)
+func (a textMapAdapter) Set(key string, value string) {
+	if a.w == nil {
+		return // not implemented on reader
+	}
+	a.w.Set(key, value)
 }
 
-func (a textMapWriterAdapter) Keys() []string {
-	// not implemented
-	return nil
+func (a textMapAdapter) Keys() []string {
+	if a.r == nil {
+		return nil // not implemented on writer
+	}
+	var ret []string
+	_ = a.r.ForeachKey(func(k, v string) error {
+		ret = append(ret, k)
+		return nil
+	})
+	return ret
 }
 
 // Inject is a part of the implementation of the OpenTracing Tracer
@@ -661,7 +681,7 @@ func (t *BridgeTracer) Inject(sm ot.SpanContext, format interface{}, carrier int
 		if !ok {
 			return ot.ErrInvalidCarrier
 		}
-		tmCarrier = textMapWriterAdapter{t: tmWriter}
+		tmCarrier = textMapAdapter{w: tmWriter}
 	}
 	fs := fakeSpan{
 		Span: noop.Span,
@@ -673,35 +693,6 @@ func (t *BridgeTracer) Inject(sm ot.SpanContext, format interface{}, carrier int
 	return nil
 }
 
-// adapt OpenTracing TextMapReader to OpenTelemetry propagation.TextMapCarrier
-type textMapReaderAdapter struct {
-	t ot.TextMapReader
-}
-
-func (a textMapReaderAdapter) Get(key string) string {
-	var ret string
-	_ = a.t.ForeachKey(func(k, v string) error {
-		if key == k {
-			ret = v
-		}
-		return nil
-	})
-	return ret
-}
-
-func (a textMapReaderAdapter) Set(key string, value string) {
-	// not implemented
-}
-
-func (a textMapReaderAdapter) Keys() []string {
-	var ret []string
-	_ = a.t.ForeachKey(func(k, v string) error {
-		ret = append(ret, k)
-		return nil
-	})
-	return ret
-}
-
 // Extract is a part of the implementation of the OpenTracing Tracer
 // interface.
 //
@@ -717,7 +708,7 @@ func (t *BridgeTracer) Extract(format interface{}, carrier interface{}) (ot.Span
 		if !ok {
 			return nil, ot.ErrInvalidCarrier
 		}
-		tmCarrier = textMapReaderAdapter{t: tmReader}
+		tmCarrier = textMapAdapter{r: tmReader}
 	}
 	ctx := t.getPropagator().Extract(context.Background(), tmCarrier)
 	baggage := baggage.FromContext(ctx)