Optimize ContentCachingRequestWrapper allocation

This commit builds on top of changes made in gh-29775 and gh-31737.
Before this change, we would allocate several byte arrays even in cases
of known request size. This could decrease performance when getting the
cached content as it requires merging several arrays and data is not
colocated in memory.

This change ensures that we create a `FastByteArrayOutputStream`
instance with the known request size so that the first allocated segment
can contain the entire content.
If the request size is not know, we will default back on the default
allocation size for the `FastByteArrayOutputStream`.

Closes gh-31834

Author: bclozel

Diff for: spring-web/src/main/java/org/springframework/web/util/ContentCachingRequestWrapper.java

@@ -56,7 +56,7 @@
  */
 public class ContentCachingRequestWrapper extends HttpServletRequestWrapper {
 
-	private final FastByteArrayOutputStream cachedContent = new FastByteArrayOutputStream();
+	private FastByteArrayOutputStream cachedContent;
 
 	@Nullable
 	private final Integer contentCacheLimit;
@@ -74,6 +74,8 @@ public class ContentCachingRequestWrapper extends HttpServletRequestWrapper {
 	 */
 	public ContentCachingRequestWrapper(HttpServletRequest request) {
 		super(request);
+		int contentLength = request.getContentLength();
+		this.cachedContent = (contentLength > 0) ? new FastByteArrayOutputStream(contentLength) : new FastByteArrayOutputStream();
 		this.contentCacheLimit = null;
 	}
 
@@ -86,6 +88,8 @@ public ContentCachingRequestWrapper(HttpServletRequest request) {
 	 */
 	public ContentCachingRequestWrapper(HttpServletRequest request, int contentCacheLimit) {
 		super(request);
+		int contentLength = request.getContentLength();
+		this.cachedContent = (contentLength > 0) ? new FastByteArrayOutputStream(contentLength) : new FastByteArrayOutputStream();
 		this.contentCacheLimit = contentCacheLimit;
 	}