Fix TODO, use sync.Map to avoid blocking calls

sdk/metric/controller/basic/controller.go

@@ -57,13 +57,8 @@ var ErrControllerStarted = fmt.Errorf("controller already started")
 // be blocked by a pull request in the basic controller.
 type Controller struct {
 	// lock protects libraries and synchronizes Start() and Stop().
-	lock sync.Mutex
-	// TODO: libraries is synchronized by lock, but could be
-	// accomplished using a sync.Map.  The SDK specification will
-	// probably require this, as the draft already states that
-	// Stop() and MeterProvider.Meter() should not block each
-	// other.
-	libraries           map[instrumentation.Library]*registry.UniqueInstrumentMeterImpl
+	lock                sync.Mutex
+	libraries           sync.Map
 	checkpointerFactory export.CheckpointerFactory
 
 	resource *resource.Resource
@@ -93,21 +88,18 @@ func (c *Controller) Meter(instrumentationName string, opts ...metric.MeterOptio
 		SchemaURL: cfg.SchemaURL(),
 	}
 
-	c.lock.Lock()
-	defer c.lock.Unlock()
-	m, ok := c.libraries[library]
+	m, ok := c.libraries.Load(library)
 	if !ok {
 		checkpointer := c.checkpointerFactory.NewCheckpointer()
-		accumulator := sdk.NewAccumulator(checkpointer)
-		m = registry.NewUniqueInstrumentMeterImpl(&accumulatorCheckpointer{
-			Accumulator:  accumulator,
-			checkpointer: checkpointer,
-			library:      library,
-		})
-
-		c.libraries[library] = m
+		m, _ = c.libraries.LoadOrStore(
+			library,
+			registry.NewUniqueInstrumentMeterImpl(&accumulatorCheckpointer{
+				Accumulator:  sdk.NewAccumulator(checkpointer),
+				checkpointer: checkpointer,
+				library:      library,
+			}))
 	}
-	return metric.WrapMeterImpl(m)
+	return metric.WrapMeterImpl(m.(*registry.UniqueInstrumentMeterImpl))
 }
 
 type accumulatorCheckpointer struct {
@@ -138,7 +130,6 @@ func New(checkpointerFactory export.CheckpointerFactory, opts ...Option) *Contro
 		}
 	}
 	return &Controller{
-		libraries:           map[instrumentation.Library]*registry.UniqueInstrumentMeterImpl{},
 		checkpointerFactory: checkpointerFactory,
 		exporter:            c.Exporter,
 		resource:            c.Resource,
@@ -251,16 +242,14 @@ func (c *Controller) collect(ctx context.Context) error {
 // accumulatorList returns a snapshot of current accumulators
 // registered to this controller.  This briefly locks the controller.
 func (c *Controller) accumulatorList() []*accumulatorCheckpointer {
-	c.lock.Lock()
-	defer c.lock.Unlock()
-
 	var r []*accumulatorCheckpointer
-	for _, entry := range c.libraries {
-		acc, ok := entry.MeterImpl().(*accumulatorCheckpointer)
+	c.libraries.Range(func(key, value interface{}) bool {
+		acc, ok := value.(*registry.UniqueInstrumentMeterImpl).MeterImpl().(*accumulatorCheckpointer)
 		if ok {
 			r = append(r, acc)
 		}
-	}
+		return true
+	})
 	return r
 }