Use snake case for nodes stats/info metric names (#53446)

The REST API uses "thread_pool" as the name of the thread pool metric.
If we use this name internally when we serialize nodes stats and info
requests, we won't need to do any fancy logic to check for and switch
out "threadPool", which was the previous internal name.

build.gradle

@@ -221,8 +221,8 @@ task verifyVersions {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = true
-final String bwc_tests_disabled_issue = "" /* place a PR link here when committing bwc changes */
+boolean bwc_tests_enabled = false
+final String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/53446" /* place a PR link here when committing bwc changes */
 if (bwc_tests_enabled == false) {
   if (bwc_tests_disabled_issue.isEmpty()) {
     throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")

server/src/main/java/org/elasticsearch/action/admin/cluster/node/info/NodesInfoRequest.java

@@ -286,7 +286,7 @@ public class NodesInfoRequest extends BaseNodesRequest<NodesInfoRequest> {
         OS("os"),
         PROCESS("process"),
         JVM("jvm"),
-        THREAD_POOL("threadPool"),
+        THREAD_POOL("thread_pool"),
         TRANSPORT("transport"),
         HTTP("http"),
         PLUGINS("plugins"),

server/src/main/java/org/elasticsearch/action/admin/cluster/node/stats/NodesStatsRequest.java

@@ -319,7 +319,7 @@ public class NodesStatsRequest extends BaseNodesRequest<NodesStatsRequest> {
         OS("os"),
         PROCESS("process"),
         JVM("jvm"),
-        THREAD_POOL("threadPool"),
+        THREAD_POOL("thread_pool"),
         FS("fs"),
         TRANSPORT("transport"),
         HTTP("http"),