refactor(metrics): distinguish different aggregator types

Separate the `toPoint` definition to distinguish the return value of
one aggregator possibly returning.

packages/opentelemetry-exporter-prometheus/test/prometheus.test.ts

@@ -21,6 +21,7 @@ import {
   Meter,
   MeterProvider,
   Point,
+  Sum,
 } from '@opentelemetry/metrics';
 import * as assert from 'assert';
 import * as http from 'http';
@@ -30,10 +31,10 @@ const mockedHrTime: HrTime = [1586347902211, 0];
 const mockedTimeMS = 1586347902211000;
 
 describe('PrometheusExporter', () => {
-  let toPoint: () => Point;
+  let toPoint: () => Point<Sum>;
   before(() => {
     toPoint = SumAggregator.prototype.toPoint;
-    SumAggregator.prototype.toPoint = function (): Point {
+    SumAggregator.prototype.toPoint = function (): Point<Sum> {
       const point = toPoint.apply(this);
       point.timestamp = mockedHrTime;
       return point;

packages/opentelemetry-metrics/src/export/aggregators/MinMaxLastSumCount.ts

@@ -14,15 +14,17 @@
  * limitations under the License.
  */
 
-import { Aggregator, Point } from '../types';
+import { Point, AggregatorKind, DistributionAggregatorType } from '../types';
 import { HrTime } from '@opentelemetry/api';
 import { hrTime } from '@opentelemetry/core';
 import { Distribution } from '../types';
 
 /**
  * Basic aggregator keeping all raw values (events, sum, max, last and min).
  */
-export class MinMaxLastSumCountAggregator implements Aggregator {
+export class MinMaxLastSumCountAggregator
+  implements DistributionAggregatorType {
+  public kind: AggregatorKind.DISTRIBUTION = AggregatorKind.DISTRIBUTION;
   private _distribution: Distribution;
   private _lastUpdateTime: HrTime = [0, 0];
 
@@ -45,7 +47,7 @@ export class MinMaxLastSumCountAggregator implements Aggregator {
     this._lastUpdateTime = hrTime();
   }
 
-  toPoint(): Point {
+  toPoint(): Point<Distribution> {
     return {
       value: this._distribution,
       timestamp: this._lastUpdateTime,

packages/opentelemetry-metrics/src/export/aggregators/Sum.ts

@@ -14,12 +14,13 @@
  * limitations under the License.
  */
 
-import { Aggregator, Point } from '../types';
+import { Point, Sum, AggregatorKind, SumAggregatorType } from '../types';
 import { HrTime } from '@opentelemetry/api';
 import { hrTime } from '@opentelemetry/core';
 
 /** Basic aggregator which calculates a Sum from individual measurements. */
-export class SumAggregator implements Aggregator {
+export class SumAggregator implements SumAggregatorType {
+  public kind: AggregatorKind.SUM = AggregatorKind.SUM;
   private _current: number = 0;
   private _lastUpdateTime: HrTime = [0, 0];
 
@@ -28,7 +29,7 @@ export class SumAggregator implements Aggregator {
     this._lastUpdateTime = hrTime();
   }
 
-  toPoint(): Point {
+  toPoint(): Point<Sum> {
     return {
       value: this._current,
       timestamp: this._lastUpdateTime,

packages/opentelemetry-metrics/src/export/aggregators/histogram.ts

@@ -14,15 +14,21 @@
  * limitations under the License.
  */
 
-import { Aggregator, Point, Histogram } from '../types';
+import {
+  HistogramAggregatorType,
+  Point,
+  Histogram,
+  AggregatorKind,
+} from '../types';
 import { HrTime } from '@opentelemetry/api';
 import { hrTime } from '@opentelemetry/core';
 
 /**
  * Basic aggregator which observes events and counts them in pre-defined buckets
  * and provides the total sum and count of all observations.
  */
-export class HistogramAggregator implements Aggregator {
+export class HistogramAggregator implements HistogramAggregatorType {
+  public kind: AggregatorKind.HISTOGRAM = AggregatorKind.HISTOGRAM;
   private _lastCheckpoint: Histogram;
   private _currentCheckpoint: Histogram;
   private _lastCheckpointTime: HrTime;
@@ -61,7 +67,7 @@ export class HistogramAggregator implements Aggregator {
     this._currentCheckpoint = this._newEmptyCheckpoint();
   }
 
-  toPoint(): Point {
+  toPoint(): Point<Histogram> {
     return {
       value: this._lastCheckpoint,
       timestamp: this._lastCheckpointTime,

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

@@ -28,12 +28,21 @@ export enum MetricKind {
   VALUE_OBSERVER,
 }
 
+/** The kind of aggregator. */
+export enum AggregatorKind {
+  SUM,
+  LAST_VALUE,
+  DISTRIBUTION,
+  HISTOGRAM,
+}
+
 /** Sum returns an aggregated sum. */
 export type Sum = number;
 
 /** LastValue returns last value. */
 export type LastValue = number;
 
+/** Distribution returns an aggregated distribution. */
 export interface Distribution {
   min: number;
   max: number;
@@ -69,6 +78,8 @@ export interface Histogram {
   count: number;
 }
 
+export type PointValueType = Sum | LastValue | Distribution | Histogram;
+
 export interface MetricRecord {
   readonly descriptor: MetricDescriptor;
   readonly labels: Labels;
@@ -102,16 +113,68 @@ export interface MetricExporter {
 /**
  * Base interface for aggregators. Aggregators are responsible for holding
  * aggregated values and taking a snapshot of these values upon export.
+ *
+ * Use {@link Aggregator} instead of this BaseAggregator.
  */
-export interface Aggregator {
+interface BaseAggregator {
+  /** The kind of the aggregator. */
+  kind: AggregatorKind;
+
   /** Updates the current with the new value. */
   update(value: number): void;
+}
+
+/** SumAggregatorType aggregate values into a {@link Sum} point type. */
+export interface SumAggregatorType extends BaseAggregator {
+  kind: AggregatorKind.SUM;
+
+  /** Returns snapshot of the current point (value with timestamp). */
+  toPoint(): Point<Sum>;
+}
+
+/**
+ * LastValueAggregatorType aggregate values into a {@link LastValue} point
+ * type.
+ */
+export interface LastValueAggregatorType extends BaseAggregator {
+  kind: AggregatorKind.LAST_VALUE;
+
+  /** Returns snapshot of the current point (value with timestamp). */
+  toPoint(): Point<LastValue>;
+}
+
+/**
+ * DistributionAggregatorType aggregate values into a {@link Distribution}
+ * point type.
+ */
+export interface DistributionAggregatorType extends BaseAggregator {
+  kind: AggregatorKind.DISTRIBUTION;
+
+  /** Returns snapshot of the current point (value with timestamp). */
+  toPoint(): Point<Distribution>;
+}
+
+/**
+ * HistogramAggregatorType aggregate values into a {@link Histogram} point
+ * type.
+ */
+export interface HistogramAggregatorType extends BaseAggregator {
+  kind: AggregatorKind.HISTOGRAM;
 
   /** Returns snapshot of the current point (value with timestamp). */
-  toPoint(): Point;
+  toPoint(): Point<Histogram>;
 }
 
-export interface Point {
-  value: Sum | LastValue | Distribution | Histogram;
+export type Aggregator =
+  | SumAggregatorType
+  | LastValueAggregatorType
+  | DistributionAggregatorType
+  | HistogramAggregatorType;
+
+/**
+ * Point represents a snapshot of aggregated values of aggregators.
+ */
+export interface Point<T extends PointValueType> {
+  value: T;
   timestamp: HrTime;
 }