Indexing instrumentation changes for auto-create indices and threadpool queue waiting

Author: rayshrey

server/src/main/java/org/opensearch/telemetry/tracing/AttributeNames.java

@@ -119,4 +119,9 @@ private AttributeNames() {
      * Search Response Total Hits
      */
     public static final String TOTAL_HITS = "total_hits";
+
+    /**
+     * Number of Indices
+     */
+    public static final String NUM_INDICES = "num_indices";
 }