[MINOR] Add requestHeaderSize debug log

## What changes were proposed in this pull request?

`requestHeaderSize` is added in apache#23090 and applies to Spark + History server UI as well. Without debug log it's hard to find out on which side what configuration is used.

In this PR I've added a log message which prints out the value.

## How was this patch tested?

Manually checked log files.

Closes apache#25045 from gaborgsomogyi/SPARK-26118.

Authored-by: Gabor Somogyi <gabor.g.somogyi@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit 0b6c2c2)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

core/src/main/scala/org/apache/spark/ui/JettyUtils.scala

@@ -357,7 +357,9 @@ private[spark] object JettyUtils extends Logging {
         (connector, connector.getLocalPort())
       }
       val httpConfig = new HttpConfiguration()
-      httpConfig.setRequestHeaderSize(conf.get(UI_REQUEST_HEADER_SIZE).toInt)
+      val requestHeaderSize = conf.get(UI_REQUEST_HEADER_SIZE).toInt
+      logDebug(s"Using requestHeaderSize: $requestHeaderSize")
+      httpConfig.setRequestHeaderSize(requestHeaderSize)
 
       // If SSL is configured, create the secure connector first.
       val securePort = sslOptions.createJettySslContextFactory().map { factory =>