[core-tracing] support sharing state between CJS and ESM (#28631)

### Packages impacted by this PR

@azure/core-tracing

### Issues associated with this PR

Resolves #28619

### Describe the problem that is addressed by this PR

If you are exporting both CommonJS and ESM forms of a package, then it
is possible for both versions to be loaded at run-time.
However, the CommonJS build is a different module from the ESM build,
and thus a different thing from the point of view of the
JavaScript interpreter in Node.js.

> https://github.com/isaacs/tshy/blob/main/README.md#dual-package-hazards

tshy handles this by building programs into separate folders and treats
"dual module hazards" as a fact of life. 

One of the hazards of dual-modules is shared module-global state.

In core-tracing we have a module-global instrumenter that is used for
hooking into and lighting up OpenTelemetry based instrumentation. In order to
ensure it works in this dual-package world we must use one of multiple-recommended
workarounds.

In this case, the tshy documentation provides a solution to this with a
well-documented path forward. This is what is implemented here.

Please refer to
https://github.com/isaacs/tshy/blob/main/README.md#module-local-state
for added context

sdk/core/core-tracing/.tshy/browser.json

@@ -5,7 +5,9 @@
     "../src/**/*.mts",
     "../src/**/*.tsx"
   ],
-  "exclude": [],
+  "exclude": [
+    ".././src/state-cjs.cts"
+  ],
   "compilerOptions": {
     "outDir": "../.tshy-build/browser"
   }

sdk/core/core-tracing/.tshy/commonjs.json

@@ -6,7 +6,8 @@
     "../src/**/*.tsx"
   ],
   "exclude": [
-    "../src/**/*.mts"
+    "../src/**/*.mts",
+    "../src/state-browser.mts"
   ],
   "compilerOptions": {
     "outDir": "../.tshy-build/commonjs"

sdk/core/core-tracing/.tshy/esm.json

@@ -5,7 +5,10 @@
     "../src/**/*.mts",
     "../src/**/*.tsx"
   ],
-  "exclude": [],
+  "exclude": [
+    ".././src/state-cjs.cts",
+    ".././src/state-browser.mts"
+  ],
   "compilerOptions": {
     "outDir": "../.tshy-build/esm"
   }

sdk/core/core-tracing/.tshy/react-native.json

@@ -5,7 +5,10 @@
     "../src/**/*.mts",
     "../src/**/*.tsx"
   ],
-  "exclude": [],
+  "exclude": [
+    ".././src/state-cjs.cts",
+    ".././src/state-browser.mts"
+  ],
   "compilerOptions": {
     "outDir": "../.tshy-build/react-native"
   }

sdk/core/core-tracing/src/instrumenter.ts

@@ -7,7 +7,9 @@ import {
   TracingContext,
   TracingSpan,
 } from "./interfaces.js";
+
 import { createTracingContext } from "./tracingContext.js";
+import { state } from "./state.js";
 
 export function createDefaultTracingSpan(): TracingSpan {
   return {
@@ -57,16 +59,13 @@ export function createDefaultInstrumenter(): Instrumenter {
   };
 }
 
-/** @internal */
-let instrumenterImplementation: Instrumenter | undefined;
-
 /**
  * Extends the Azure SDK with support for a given instrumenter implementation.
  *
  * @param instrumenter - The instrumenter implementation to use.
  */
 export function useInstrumenter(instrumenter: Instrumenter): void {
-  instrumenterImplementation = instrumenter;
+  state.instrumenterImplementation = instrumenter;
 }
 
 /**
@@ -75,8 +74,8 @@ export function useInstrumenter(instrumenter: Instrumenter): void {
  * @returns The currently set instrumenter
  */
 export function getInstrumenter(): Instrumenter {
-  if (!instrumenterImplementation) {
-    instrumenterImplementation = createDefaultInstrumenter();
+  if (!state.instrumenterImplementation) {
+    state.instrumenterImplementation = createDefaultInstrumenter();
   }
-  return instrumenterImplementation;
+  return state.instrumenterImplementation;
 }

sdk/core/core-tracing/src/state-browser.mts

@@ -0,0 +1,11 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+import { Instrumenter } from "./interfaces.js";
+
+/**
+ * Browser-only implementation of the module's state. The browser esm variant will not load the commonjs state, so we do not need to share state between the two.
+ */
+export const state = {
+  instrumenterImplementation: undefined as Instrumenter | undefined,
+};

sdk/core/core-tracing/src/state-cjs.cts

@@ -0,0 +1,11 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+/**
+ * @internal
+ *
+ * Holds the singleton instrumenter, to be shared across CJS and ESM imports.
+ */
+export const state = {
+  instrumenterImplementation: undefined,
+};

sdk/core/core-tracing/src/state.ts

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+import { Instrumenter } from "./interfaces.js";
+// @ts-expect-error The recommended approach to sharing module state between ESM and CJS.
+// See https://github.com/isaacs/tshy/blob/main/README.md#module-local-state for additional information.
+import { state as cjsState } from "../commonjs/state.js";
+
+/**
+ * Defines the shared state between CJS and ESM by re-exporting the CJS state.
+ */
+export const state = cjsState as {
+  instrumenterImplementation: Instrumenter | undefined;
+};

sdk/core/core-tracing/vitest.config.ts

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 import { defineConfig } from "vitest/config";
+import { resolve } from "node:path";
 
 export default defineConfig({
   test: {
@@ -13,6 +14,9 @@ export default defineConfig({
       toFake: ["setTimeout", "Date"],
     },
     watch: false,
+    alias: {
+      "../commonjs/state.js": resolve("./src/state-cjs.cts"),
+    },
     include: ["test/**/*.spec.ts"],
     exclude: ["test/**/browser/*.spec.ts"],
     coverage: {