feat(metrics): Add types and base implementation (jaegertracing#310)

* feat(metrics): add types and base implementation

* fix: styling

* Update packages/opentelemetry-metrics/src/export/types.ts

Make sum optional

* fix: make sum optional

* fix: use HrTime interface

Author: danielkhan

packages/opentelemetry-metrics/src/export/MetricProducerManager.ts

@@ -0,0 +1,78 @@
+/*!
+ * Copyright 2019, OpenTelemetry Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { MetricProducer, MetricProducerManager } from './types';
+
+/**
+ * Keeps a set of MetricProducer that is used by exporters to determine the
+ * metrics that need to be exported.
+ */
+class BaseMetricProducerManager implements MetricProducerManager {
+  /** Singleton instance */
+  private static _singletonInstance: MetricProducerManager;
+
+  private _metricProducers: Set<MetricProducer> = new Set<MetricProducer>();
+
+  /** Gets the instance. */
+  static get instance(): MetricProducerManager {
+    return this._singletonInstance || (this._singletonInstance = new this());
+  }
+
+  /**
+   * Adds the MetricProducer to the manager if it is not already present.
+   *
+   * @param metricProducer The MetricProducer to be added to the manager.
+   */
+  add(metricProducer: MetricProducer): void {
+    // TODO: How to we handle this validation - throws in OpenCensus?
+    // validateNotNull(metricProducer, 'metricProducer');
+    if (!this._metricProducers.has(metricProducer)) {
+      this._metricProducers.add(metricProducer);
+    }
+  }
+
+  /**
+   * Removes the MetricProducer to the manager if it is present.
+   *
+   * @param metricProducer The MetricProducer to be removed from the manager.
+   */
+  remove(metricProducer: MetricProducer): void {
+    // TODO: How to we handle this validation - throws in OpenCensus?
+    // validateNotNull(metricProducer, 'metricProducer');
+    this._metricProducers.delete(metricProducer);
+  }
+
+  /**
+   * Clears all MetricProducers.
+   */
+  removeAll(): void {
+    this._metricProducers.clear();
+  }
+
+  /**
+   * Returns all registered MetricProducers that should be exported.
+   *
+   * This method should be used by any metrics exporter that automatically
+   * exports data for MetricProducer registered with the MetricProducerManager.
+   *
+   * @return {Set<MetricProducer>} The Set of MetricProducers.
+   */
+  getAllMetricProducer(): Set<MetricProducer> {
+    return this._metricProducers;
+  }
+}
+
+export const metricProducerManagerInstance = BaseMetricProducerManager.instance;