Dinamically add metric constructors to metric map

This is nice because it means metrics will work exactly like plugins in
the sense that they are only added to the bundle if they are used.

I am not entirely sure about this change, because it means that if a
given metric is in the database, but was not added to the metric map
(think if a metric is removed from the code, but has not been collected
in a ping once the new code lands) it will be read as invalid by Glean.

Author: brizental

glean/src/core/context.ts

@@ -8,6 +8,8 @@ import type EventsDatabase from "./metrics/events_database/index.js";
 import type PingsDatabase from "./pings/database.js";
 import type ErrorManager from "./error/index.js";
 import type Platform from "../platform/index.js";
+import type { Metric } from "./metrics/metric.js";
+import type { JSONValue } from "./utils.js";
 import Dispatcher from "./dispatcher.js";
 import log, { LoggingLevel } from "./log.js";
 
@@ -45,6 +47,13 @@ export class Context {
   private _initialized = false;
   // Whether or not Glean is in testing mode.
   private _testing = false;
+  // A map of metric types and their constructors.
+  // This map is dinamically filled everytime a metric type is constructed.
+  //
+  // If a metric is not on this map it cannot be serialized from the database.
+  private _supportedMetrics: {
+    [type: string]: new (v: unknown) => Metric<JSONValue, JSONValue>
+  } = {};
 
   // The moment the current Glean.js session started.
   private _startTime: Date;
@@ -242,4 +251,27 @@ export class Context {
   static isPlatformSet(): boolean {
     return !!Context.instance._platform;
   }
+
+  static getSupportedMetric(type: string): (new (v: unknown) => Metric<JSONValue, JSONValue>) | undefined {
+    return Context.instance._supportedMetrics[type];
+  }
+
+  /**
+   * Adds a new constructor to the supported metrics map.
+   *
+   * If the metric map already contains this constructor, this is a no-op.
+   *
+   * @param type A string identifying the given metric type.
+   * @param ctn The metric constructor.
+   */
+  static addSupportedMetric(
+    type: string,
+    ctn: new (v: unknown) => Metric<JSONValue, JSONValue>
+  ): void {
+    if (type in Context.instance._supportedMetrics) {
+      return;
+    }
+
+    Context.instance._supportedMetrics[type] = ctn;
+  }
 }

glean/src/core/glean.ts

@@ -526,6 +526,10 @@ class Glean {
     uploadEnabled = true,
     config?: ConfigurationInterface
   ): Promise<void> {
+    // Core metrics need tp be re-initialized so that
+    // the supportedMetrics map is re-created.
+    Glean.instance._coreMetrics = new CoreMetrics();
+
     Context.testing = true;
 
     Glean.setPlatform(TestPlatform);

glean/src/core/metrics/index.ts

@@ -3,9 +3,10 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 import type { JSONValue} from "../utils.js";
+import { isUndefined, testOnly } from "../utils.js";
 import type { Lifetime } from "./lifetime.js";
 import type { ErrorType } from "../error/error_type.js";
-import { isUndefined, testOnly } from "../utils.js";
+import type { Metric } from "./metric.js";
 import { getValidDynamicLabel } from "./types/labeled.js";
 import { Context } from "../context.js";
 
@@ -53,7 +54,14 @@ export abstract class MetricType implements CommonMetricData {
   readonly disabled: boolean;
   dynamicLabel?: string;
 
-  constructor(type: string, meta: CommonMetricData) {
+  constructor(
+    type: string,
+    meta: CommonMetricData,
+    metricCtn?: new (v: unknown) => Metric<JSONValue, JSONValue>
+  ) {
+    if (metricCtn) {
+      Context.addSupportedMetric(type, metricCtn);
+    }
     this.type = type;
 
     this.name = meta.name;

glean/src/core/metrics/types/boolean.ts

@@ -30,7 +30,7 @@ export class BooleanMetric extends Metric<boolean, boolean> {
  */
 class BooleanMetricType extends MetricType {
   constructor(meta: CommonMetricData) {
-    super("boolean", meta);
+    super("boolean", meta, BooleanMetric);
   }
 
   /**

glean/src/core/metrics/types/counter.ts

@@ -43,7 +43,7 @@ export class CounterMetric extends Metric<number, number> {
  */
 class CounterMetricType extends MetricType {
   constructor(meta: CommonMetricData) {
-    super("counter", meta);
+    super("counter", meta, CounterMetric);
   }
 
   /**

glean/src/core/metrics/types/datetime.ts

@@ -168,7 +168,7 @@ class DatetimeMetricType extends MetricType {
   timeUnit: TimeUnit;
 
   constructor(meta: CommonMetricData, timeUnit: string) {
-    super("datetime", meta);
+    super("datetime", meta, DatetimeMetric);
     this.timeUnit = timeUnit as TimeUnit;
   }
 

glean/src/core/metrics/types/quantity.ts

@@ -41,7 +41,7 @@ export class QuantityMetric extends Metric<number, number> {
  */
 class QuantityMetricType extends MetricType {
   constructor(meta: CommonMetricData) {
-    super("quantity", meta);
+    super("quantity", meta, QuantityMetric);
   }
 
   /**

glean/src/core/metrics/types/rate.ts

@@ -60,7 +60,7 @@ export class RateMetric extends Metric<Rate, Rate> {
  */
 class RateMetricType extends MetricType {
   constructor(meta: CommonMetricData) {
-    super("rate", meta);
+    super("rate", meta, RateMetric);
   }
 
   /**

glean/src/core/metrics/types/string.ts

@@ -41,7 +41,7 @@ export class StringMetric extends Metric<string, string> {
  */
 class StringMetricType extends MetricType {
   constructor(meta: CommonMetricData) {
-    super("string", meta);
+    super("string", meta, StringMetric);
   }
 
   /**

glean/src/core/metrics/types/string_list.ts

@@ -51,7 +51,7 @@ export class StringListMetric extends Metric<string[], string[]> {
  */
 class StringListMetricType extends MetricType {
   constructor(meta: CommonMetricData) {
-    super("string_list", meta);
+    super("string_list", meta, StringListMetric);
   }
 
   /**