[ML] Explain Log Rate Spikes: Fix slowness of flushFix random paylo…

…ad generation. (#160645)

The random payload for the proxy flushing fix was regenerated for every
push to the stream. This turned out to be quite slow. This PR updates
the logic to create the payload only once and reuse it for every push.

Note: Further testing in Cloud showed the differences there are not as
big as in my local testing, might be related to entropy for
`crypto.randomBytes`.

x-pack/packages/ml/aiops_utils/src/stream_factory.ts

@@ -73,6 +73,9 @@ export function streamFactory<T = unknown>(
 ): StreamFactoryReturnType<T> {
   let streamType: StreamType;
   const isCompressed = compressOverride && acceptCompression(headers);
+  const flushPayload = flushFix
+    ? crypto.randomBytes(FLUSH_PAYLOAD_SIZE).toString('hex')
+    : undefined;
 
   const stream = isCompressed ? zlib.createGzip() : new UncompressedResponseStream();
 
@@ -149,9 +152,7 @@ export function streamFactory<T = unknown>(
           ? `${JSON.stringify({
               ...d,
               // This is a temporary fix for response streaming with proxy configurations that buffer responses up to 4KB in size.
-              ...(flushFix
-                ? { flushPayload: crypto.randomBytes(FLUSH_PAYLOAD_SIZE).toString('hex') }
-                : {}),
+              ...(flushFix ? { flushPayload } : {}),
             })}${DELIMITER}`
           : d;