Add API for registering a global instance.

lib/sdk.dart

@@ -1,5 +1,6 @@
 export 'src/sdk/common/attribute.dart' show Attribute;
-export 'src/sdk/open_telemetry.dart' show OpenTelemetrySdk;
+export 'src/sdk/open_telemetry.dart'
+    show registerGlobalTracerProvider, globalTracerProvider;
 export 'src/sdk/trace/exporters/collector_exporter.dart' show CollectorExporter;
 export 'src/sdk/trace/exporters/console_exporter.dart' show ConsoleExporter;
 export 'src/sdk/trace/propagation/w3c_trace_context_propagator.dart'

lib/src/sdk/open_telemetry.dart

@@ -1,35 +1,16 @@
-import 'package:opentelemetry/sdk.dart';
+import '../../sdk.dart';
 
-class OpenTelemetrySdk {
-  static final OpenTelemetrySdk _globalOpenTelemetry =
-      OpenTelemetrySdk._global();
-  TracerProvider _tracerProvider;
+final TracerProvider _noopTracerProvider = TracerProvider();
+TracerProvider _tracerProvider = _noopTracerProvider;
 
-  OpenTelemetrySdk._global();
-
-  /// Returns a new [OpenTelemetrySdk] and registers it as the global
-  /// [OpenTelemetrySdk]. Future calls to getter globalOpenTelemetry will return
-  /// the registered [OpenTelemetrySdk] instance.
-  ///
-  /// This should be called once as early as possible in your application
-  /// initialization logic.  A [StateError] will be thrown if this method is
-  /// attempted to be called multiple times in the lifecycle of an
-  /// application - ensure you have only one for use as the global instance.
-  factory OpenTelemetrySdk.global(TracerProvider tracerProvider) {
-    if (_globalOpenTelemetry._tracerProvider != null) {
-      throw StateError(
-          'A global instance of OpenTelemetry has already been created. '
-          'OpenTelemetry.global must be called only once before any calls '
-          'to getter globalOpenTelemetry.');
-    }
-
-    _globalOpenTelemetry._tracerProvider = tracerProvider;
-
-    return _globalOpenTelemetry;
+void registerGlobalTracerProvider(TracerProvider tracerProvider) {
+  if (_tracerProvider != _noopTracerProvider) {
+    throw StateError('A global TracerProvider has already been created. '
+        'registerGlobalTracerProvider must be called only once before any '
+        'calls to getter globalTracerProvider.');
   }
 
-  OpenTelemetrySdk(this._tracerProvider);
-
-  static OpenTelemetrySdk get globalOpenTelemetry => _globalOpenTelemetry;
-  TracerProvider get tracerProvider => _tracerProvider;
+  _tracerProvider = tracerProvider;
 }
+
+TracerProvider get globalTracerProvider => _tracerProvider;

lib/src/sdk/trace/tracer_provider.dart

@@ -2,9 +2,7 @@ import '../../api/instrumentation_library.dart' as version_api;
 import '../../api/span_processors/span_processor.dart';
 import '../../api/trace/tracer_provider.dart' as api;
 import '../instrumentation_library.dart';
-import 'exporters/console_exporter.dart';
 import 'id_generator.dart';
-import 'span_processors/simple_processor.dart';
 import 'tracer.dart';
 
 /// A registry for creating named [Tracer]s.
@@ -16,7 +14,7 @@ class TracerProvider implements api.TracerProvider {
   IdGenerator _idGenerator;
 
   TracerProvider({List<SpanProcessor> processors, IdGenerator idGenerator}) {
-    _processors = processors ?? [SimpleSpanProcessor(ConsoleExporter())];
+    _processors = processors ?? [];  // Default to a no-op TracerProvider.
     _idGenerator = idGenerator ?? IdGenerator();
     // TODO: O11Y-1027: Per spec, a Sampler defaulted to ParentBased(root=AlwaysOn) added here.
   }