chore: add call incoming webhook rate limit metric (#6252)

Follow up to https://github.com/Unleash/unleash/pull/6248 - Add this
rate limit to metrics.


![image](https://github.com/Unleash/unleash/assets/14320932/6b458f5d-2e13-4ac4-8331-7950e7784585)

src/lib/metrics.ts

@@ -355,6 +355,15 @@ export default class MetricsMonitor {
                 rateLimits
                     .labels({ endpoint: '/auth/simple', method: 'POST' })
                     .set(config.rateLimiting.simpleLoginMaxPerMinute);
+                rateLimits
+                    .labels({
+                        endpoint: '/api/incoming-webhook/:name',
+                        method: 'POST',
+                    })
+                    .set(
+                        config.rateLimiting.callIncomingWebhookMaxPerSecond *
+                            60,
+                    );
             } catch (e) {}
         }