### Description

Implement health indicator for shard allocation.
This indicator should have same or similar behavior to existing health endpoint.

Aspects to be considered:
* nodes scheduled for reboot should not cause cluster to go yellow (but could cause it go red)
* huge number of unallocated shards should not cause response bloat.
* consider showing allocation explain if possible

Rel: TBD
server/src/main/java/org/elasticsearch/node/Node.java
injector.getInstance(ClusterService.class).stop();
        injector.getInstance(NodeConnectionsService.class).stop();
        injector.getInstance(FsHealthService.class).stop();
        nodeService.getMonitorService().stop();
        injector.getInstance(GatewayService.class).stop();
        injector.getInstance(SearchService.class).stop();
        injector.getInstance(TransportService.class).stop();
        pluginLifecycleComponents.forEach(LifecycleComponent::stop);
        
        
        injector.getInstance(IndicesService.class).stop();
        logger.info("stopped");
        return this;
    }
    
    
    
    
    @Override
    public synchronized void close() throws IOException {
        synchronized (lifecycle) {
            if (lifecycle.started()) {
                stop();
            }
            if (lifecycle.moveToClosed() == false) {
                return;
            }
        }
        logger.info("closing ...");
        List<Closeable> toClose = new ArrayList<>();