add JSON serialization

src/sdam/topology_description.ts

@@ -1,4 +1,4 @@
-import type { ObjectId } from '../bson';
+import { EJSON, type ObjectId } from '../bson';
 import * as WIRE_CONSTANTS from '../cmap/wire_protocol/constants';
 import { type MongoError, MongoRuntimeError } from '../error';
 import { compareObjectId, shuffle } from '../utils';
@@ -342,6 +342,10 @@ export class TopologyDescription {
   hasServer(address: string): boolean {
     return this.servers.has(address);
   }
+
+  toJSON() {
+    return EJSON.serialize(this);
+  }
 }
 
 function topologyTypeForServerType(serverType: ServerType): TopologyType {

test/integration/server-discovery-and-monitoring/topology_description.test.ts

@@ -87,7 +87,7 @@ describe('TopologyDescription (integration tests)', function () {
       expect(description).to.exist;
 
       const { servers } = JSON.parse(JSON.stringify(description));
-      expect(Object.keys(servers).length > 0).to.be.true;
+      expect(Object.keys(servers).length > 0, '`servers` stringified with no servers.').to.be.true;
     });
   });
 });