test: add load test for session creation (#8088)

# Which Problems Are Solved

Extends load tests by testing session creation.

# How the Problems Are Solved

The test creates a session including a check for user id.

# Additional Context

- part of https://github.com/zitadel/zitadel/issues/7639

load-test/src/session.ts

@@ -0,0 +1,42 @@
+import { Trend } from 'k6/metrics';
+import { Org } from './org';
+import http from 'k6/http';
+import url from './url';
+import { check } from 'k6';
+import { User } from './user';
+
+export type Session = {
+  id: string;
+};
+
+const addSessionTrend = new Trend('session_add_session_duration', true);
+export function createSession(user: User, org: Org, accessToken: string): Promise<Session> {
+  return new Promise((resolve, reject) => {
+    let response = http.asyncRequest(
+      'POST',
+      url('/v2beta/sessions'),
+      JSON.stringify({
+        checks: {
+            user: {
+                userId: user.userId,
+            }
+        }
+      }),
+      {
+        headers: {
+          authorization: `Bearer ${accessToken}`,
+          'Content-Type': 'application/json',
+          'x-zitadel-orgid': org.organizationId,
+        },
+      },
+    );
+    response.then((res) => {
+      check(res, {
+        'add Session status ok': (r) => r.status === 201,
+      }) || reject(`unable to add Session status: ${res.status} body: ${res.body}`);
+
+      addSessionTrend.add(res.timings.duration);
+      resolve(res.json() as Session);
+    });
+  });
+}