refactor(exporter-prometheus): remove unnecessary isNaN() check (open-telemetry#5377)

Author: cjihrig

experimental/CHANGELOG.md

@@ -32,6 +32,7 @@ All notable changes to experimental packages in this project will be documented
 
 * chore(instrumentation-grpc): remove unused findIndex() function [#5372](https://github.com/open-telemetry/opentelemetry-js/pull/5372) @cjihrig
 * refactor(otlp-exporter-base): remove unnecessary isNaN() checks [#5374](https://github.com/open-telemetry/opentelemetry-js/pull/5374) @cjihrig
+* refactor(exporter-prometheus): remove unnecessary isNaN() check [#5377](https://github.com/open-telemetry/opentelemetry-js/pull/5377) @cjihrig
 
 ## 0.57.0
 

experimental/packages/opentelemetry-exporter-prometheus/src/PrometheusSerializer.ts

@@ -104,15 +104,12 @@ function enforcePrometheusNamingConvention(
 }
 
 function valueString(value: number) {
-  if (Number.isNaN(value)) {
-    return 'NaN';
-  } else if (!Number.isFinite(value)) {
-    if (value < 0) {
-      return '-Inf';
-    } else {
-      return '+Inf';
-    }
+  if (value === Infinity) {
+    return '+Inf';
+  } else if (value === -Infinity) {
+    return '-Inf';
   } else {
+    // Handle finite numbers and NaN.
     return `${value}`;
   }
 }

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

@@ -639,6 +639,7 @@ describe('PrometheusSerializer', () => {
     it('should serialize non-finite values', async () => {
       const serializer = new PrometheusSerializer();
       const cases = [
+        [NaN, 'NaN'],
         [-Infinity, '-Inf'],
         [+Infinity, '+Inf'],
       ] as [number, string][];