Add docstring for workflowDiagnosticsEnabled feature flag resolver (#977)

* Add env variable for enabling workflow diagnostics, and use it in the workflowDiagnosticsEnabled feature flag resolver
* Add docstring to workflowDiagnosticsEnabled resolver, detailing required server versions for full & partial support.

Author: adhityamamallan

src/config/dynamic/resolvers/workflow-diagnostics-enabled.ts

@@ -1,3 +1,16 @@
+/**
+ * Returns whether workflow diagnostics APIs and UI are enabled.
+ *
+ * To enable workflow diagnostics, set the CADENCE_WORKFLOW_DIAGNOSTICS_ENABLED env variable to true.
+ * For further customization, override the implementation of this resolver.
+ *
+ * Server version behaviour:
+ * - \>= 1.3.1: Diagnostics APIs and UI will work as expected.
+ * - \>= 1.2.13 & < 1.3.1: Diagnostics APIs will work, but the format may not be as expected; only raw JSON will be displayed.
+ * - < 1.2.13: Diagnostics APIs will return a GRPC unimplemented error (maps to HTTP 404 in the client).
+ *
+ * @returns {Promise<boolean>} Whether workflow diagnostics are enabled.
+ */
 export default async function workflowDiagnosticsEnabled(): Promise<boolean> {
-  return false;
+  return process.env.CADENCE_WORKFLOW_DIAGNOSTICS_ENABLED === 'true';
 }