Rename blobName to fileName in blob cache metrics (#134357)

We record the lucene file extention when fetching data from the blob
store. The method argument was named `blobName` which was confusing.
This changes the argument name to `fileName` for clarity.

Author: andreidan

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/BlobCacheMetrics.java

@@ -170,28 +170,28 @@ public LongHistogram getCacheMissLoadTimes() {
     /**
      * Record the various cache population metrics after a chunk is copied to the cache
      *
-     * @param blobName The file that was requested and triggered the cache population.
+     * @param fileName The actual (lucene) file that's requested from the blob location
      * @param bytesCopied The number of bytes copied
      * @param copyTimeNanos The time taken to copy the bytes in nanoseconds
      * @param cachePopulationReason The reason for the cache being populated
      * @param cachePopulationSource The source from which the data is being loaded
      */
     public void recordCachePopulationMetrics(
-        String blobName,
+        String fileName,
         int bytesCopied,
         long copyTimeNanos,
         CachePopulationReason cachePopulationReason,
         CachePopulationSource cachePopulationSource
     ) {
-        LuceneFilesExtensions luceneFilesExtensions = LuceneFilesExtensions.fromFile(blobName);
-        String blobFileExtension = luceneFilesExtensions != null ? luceneFilesExtensions.getExtension() : NON_LUCENE_EXTENSION_TO_RECORD;
+        LuceneFilesExtensions luceneFilesExtensions = LuceneFilesExtensions.fromFile(fileName);
+        String luceneFileExt = luceneFilesExtensions != null ? luceneFilesExtensions.getExtension() : NON_LUCENE_EXTENSION_TO_RECORD;
         Map<String, Object> metricAttributes = Map.of(
             CACHE_POPULATION_REASON_ATTRIBUTE_KEY,
             cachePopulationReason.name(),
             CACHE_POPULATION_SOURCE_ATTRIBUTE_KEY,
             cachePopulationSource.name(),
             LUCENE_FILE_EXTENSION_ATTRIBUTE_KEY,
-            blobFileExtension
+            luceneFileExt
         );
         assert bytesCopied > 0 : "We shouldn't be recording zero-sized copies";
         cachePopulationBytes.incrementBy(bytesCopied, metricAttributes);