fix(types): make MetricType a string union matching runtime values

CHANGELOG.md

@@ -15,6 +15,11 @@ This release marks our first release under the Prometheus umbrella.
 - Metric internal storage ('hashMap') changed to a separate object, LabelMap. If you have
   subclassed the built-in metric types you may need to adjust your code.
 - Counter Exemplars now report the value rather than the delta
+- TypeScript: `MetricType` is now a string union matching the runtime values
+  (`'counter' | 'gauge' | 'histogram' | 'summary'`) instead of a numeric enum that had no
+  runtime object. Value-style uses such as `MetricType.Counter` (which threw at runtime)
+  no longer compile; compare against the string literals instead. Under
+  `verbatimModuleSyntax`, import it with `import type`.
 
 ### Changed
 

index.d.ts

@@ -270,12 +270,11 @@ export type Metric<T extends string = NoLabelNameType> =
  */
 export type Aggregator = 'omit' | 'sum' | 'first' | 'min' | 'max' | 'average';
 
-export enum MetricType {
-	Counter,
-	Gauge,
-	Histogram,
-	Summary,
-}
+/**
+ * The metric type reported in metric objects, such as those returned by
+ * `Registry#getMetricsAsJSON()`. Matches the runtime string values.
+ */
+export type MetricType = 'counter' | 'gauge' | 'histogram' | 'summary';
 
 type CollectFunction<T> = (this: T) => void | Promise<void>;
 

test/typescript.ts

@@ -18,6 +18,7 @@ import {
 	Registry,
 	MetricObject,
 	MetricObjectWithValues,
+	MetricType,
 	MetricValue,
 	MetricValueWithName,
 } from '../index';
@@ -71,3 +72,38 @@ async function metricObjectTypesAreExported() {
 	void named;
 }
 void metricObjectTypesAreExported;
+
+// MetricType matches the runtime strings reported in metric metadata, so a
+// reported type can be compared against a literal and narrowed without casts.
+// The switch fails to compile if the union and this member list ever drift
+// apart, in either direction.
+async function metricTypeMatchesRuntimeStrings() {
+	const t: MetricType = 'counter';
+	void t;
+
+	const [first] = await registry.getMetricsAsJSON();
+	if (first !== undefined && first.type === 'counter') {
+		const narrowed: 'counter' = first.type;
+		void narrowed;
+	}
+
+	const lockMembers = (type: MetricType): string => {
+		switch (type) {
+			case 'counter':
+			case 'gauge':
+			case 'histogram':
+			case 'summary':
+				return type;
+			default: {
+				const missing: never = type;
+				return missing;
+			}
+		}
+	};
+	void lockMembers;
+
+	// @ts-expect-error MetricType is a type-only string union with no runtime
+	// object (#336), so value-style access must not compile.
+	void MetricType.Counter;
+}
+void metricTypeMatchesRuntimeStrings;