[SPARK-48011][CORE] Store LogKey name as a value to avoid generating …

…new string instances

### What changes were proposed in this pull request?

Store LogKey name as a value to avoid generating new string instances
### Why are the changes needed?

To save memory usage on getting the names of `LogKey`s.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Existing tests

### Was this patch authored or co-authored using generative AI tooling?

No

Closes apache#46249 from gengliangwang/addKeyName.

Authored-by: Gengliang Wang <gengliang@apache.org>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

common/utils/src/main/scala/org/apache/spark/internal/LogKey.scala

@@ -16,10 +16,14 @@
  */
 package org.apache.spark.internal
 
+import java.util.Locale
+
 /**
  * All structured logging `keys` used in `MDC` must be extends `LogKey`
  */
-trait LogKey
+trait LogKey {
+  val name: String = this.toString.toLowerCase(Locale.ROOT)
+}
 
 /**
  * Various keys used for mapped diagnostic contexts(MDC) in logging.

common/utils/src/main/scala/org/apache/spark/internal/Logging.scala

@@ -17,8 +17,6 @@
 
 package org.apache.spark.internal
 
-import java.util.Locale
-
 import scala.jdk.CollectionConverters._
 
 import org.apache.logging.log4j.{CloseableThreadContext, Level, LogManager}
@@ -110,7 +108,7 @@ trait Logging {
         val value = if (mdc.value != null) mdc.value.toString else null
         sb.append(value)
         if (Logging.isStructuredLoggingEnabled) {
-          context.put(mdc.key.toString.toLowerCase(Locale.ROOT), value)
+          context.put(mdc.key.name, value)
         }
 
         if (processedParts.hasNext) {