### 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
toClose.add(() -> stopWatch.stop().start("search"));
        toClose.add(injector.getInstance(SearchService.class));
        toClose.add(() -> stopWatch.stop().start("transport"));
        toClose.add(injector.getInstance(TransportService.class));
        for (LifecycleComponent plugin : pluginLifecycleComponents) {
            toClose.add(() -> stopWatch.stop().start("plugin(" + plugin.getClass().getName() + ")"));
            toClose.add(plugin);
        }
        toClose.addAll(pluginsService.filterPlugins(Plugin.class));
        toClose.add(() -> stopWatch.stop().start("script"));
        toClose.add(injector.getInstance(ScriptService.class));
        toClose.add(() -> stopWatch.stop().start("thread_pool"));
        toClose.add(() -> injector.getInstance(ThreadPool.class).shutdown());
        
        
        
        toClose.add(() -> stopWatch.stop().start("gateway_meta_state"));
        toClose.add(injector.getInstance(GatewayMetaState.class));
        toClose.add(() -> stopWatch.stop().start("node_environment"));
        toClose.add(injector.getInstance(NodeEnvironment.class));
        toClose.add(stopWatch::stop);
        if (logger.isTraceEnabled()) {
            toClose.add(() -> logger.trace("Close times for each service:\n{}", stopWatch.prettyPrint()));
        }
        IOUtils.close(toClose);
        logger.info("closed");
    }
    /**
     * Wait for this node to be effectively closed.
     */