fix: cache GitHub App ID to reduce SSM calls

Cache the GitHub App ID at module level to avoid repeated SSM
parameter fetches on every rate limit metric update. The value is
now fetched once per Lambda execution context and reused across
invocations.

Author: Brend-Smits

lambdas/functions/control-plane/src/github/rate-limit.ts

@@ -4,6 +4,16 @@ import { MetricUnit } from '@aws-lambda-powertools/metrics';
 import yn from 'yn';
 import { getParameter } from '@aws-github-runner/aws-ssm-util';
 
+// Cache the app ID to avoid repeated SSM calls across Lambda invocations
+let appIdPromise: Promise<string> | null = null;
+
+async function getAppId(): Promise<string> {
+  if (!appIdPromise) {
+    appIdPromise = getParameter(process.env.PARAMETER_GITHUB_APP_ID_NAME);
+  }
+  return appIdPromise;
+}
+
 export async function metricGitHubAppRateLimit(headers: ResponseHeaders): Promise<void> {
   try {
     const remaining = parseInt(headers['x-ratelimit-remaining'] as string);
@@ -13,7 +23,7 @@ export async function metricGitHubAppRateLimit(headers: ResponseHeaders): Promis
 
     const updateMetric = yn(process.env.ENABLE_METRIC_GITHUB_APP_RATE_LIMIT);
     if (updateMetric) {
-      const appId = await getParameter(process.env.PARAMETER_GITHUB_APP_ID_NAME);
+      const appId = await getAppId();
       const metric = createSingleMetric('GitHubAppRateLimitRemaining', MetricUnit.Count, remaining, {
         AppId: appId,
       });