### 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(jvm.getUptime(), deserializedJvm.getUptime());
                    if (jvm.getBufferPools() == null) {
                        assertNull(deserializedJvm.getBufferPools());
                    } else {
                        assertEquals(jvm.getBufferPools().size(), deserializedJvm.getBufferPools().size());
                        for (int i = 0; i < jvm.getBufferPools().size(); i++) {
                            JvmStats.BufferPool bufferPool = jvm.getBufferPools().get(i);
                            JvmStats.BufferPool deserializedBufferPool = deserializedJvm.getBufferPools().get(i);
                            assertEquals(bufferPool.getName(), deserializedBufferPool.getName());
                            assertEquals(bufferPool.getCount(), deserializedBufferPool.getCount());
                            assertEquals(bufferPool.getTotalCapacity(), deserializedBufferPool.getTotalCapacity());
                            assertEquals(bufferPool.getUsed(), deserializedBufferPool.getUsed());
                        }
                    }
                }
                if (nodeStats.getThreadPool() == null) {
                    assertNull(deserializedNodeStats.getThreadPool());
                } else {
                    assertNotSame(nodeStats.getThreadPool(), deserializedNodeStats.getThreadPool());
                    assertEquals(nodeStats.getThreadPool(), deserializedNodeStats.getThreadPool());
                }
                FsInfo fs = nodeStats.getFs();
                FsInfo deserializedFs = deserializedNodeStats.getFs();
                if (fs == null) {
                    assertNull(deserializedFs);
                } else {
                    assertEquals(fs.getTimestamp(), deserializedFs.getTimestamp());
                    assertEquals(fs.getTotal().getAvailable(), deserializedFs.getTotal().getAvailable());
                    assertEquals(fs.getTotal().getTotal(), deserializedFs.getTotal().getTotal());
                    assertEquals(fs.getTotal().getFree(), deserializedFs.getTotal().getFree());