### Description

From the nodes stats API, we should extract the `thread_pool` stats into their own endpoint. This endpoint can then be non-node-centric, and present a global view of the number of completed, rejected, and failed operations for each of the threadpools in the system.
server/src/test/java/org/elasticsearch/action/admin/cluster/node/stats/NodeStatsTests.java
);
                    assertEquals(
                        nodeStats.getOs().getCgroup().getCpuAcctUsageNanos(),
                        deserializedNodeStats.getOs().getCgroup().getCpuAcctUsageNanos()
                    );
                    assertEquals(
                        nodeStats.getOs().getCgroup().getCpuControlGroup(),
                        deserializedNodeStats.getOs().getCgroup().getCpuControlGroup()
                    );
                    assertEquals(
                        nodeStats.getOs().getCgroup().getCpuCfsPeriodMicros(),
                        deserializedNodeStats.getOs().getCgroup().getCpuCfsPeriodMicros()
                    );
                    assertEquals(
                        nodeStats.getOs().getCgroup().getCpuCfsQuotaMicros(),
                        deserializedNodeStats.getOs().getCgroup().getCpuCfsQuotaMicros()
                    );
                    assertEquals(
                        nodeStats.getOs().getCgroup().getCpuStat().getNumberOfElapsedPeriods(),
                        deserializedNodeStats.getOs().getCgroup().getCpuStat().getNumberOfElapsedPeriods()
                    );
                    assertEquals(
                        nodeStats.getOs().getCgroup().getCpuStat().getNumberOfTimesThrottled(),
                        deserializedNodeStats.getOs().getCgroup().getCpuStat().getNumberOfTimesThrottled()
                    );
                    assertEquals(
                        nodeStats.getOs().getCgroup().getCpuStat().getTimeThrottledNanos(),
                        deserializedNodeStats.getOs().getCgroup().getCpuStat().getTimeThrottledNanos()
                    );
                    assertEquals(