### 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
IndexStorePlugin.class
            ).stream().map(IndexStorePlugin::getIndexFoldersDeletionListeners).flatMap(List::stream).collect(Collectors.toList());
            final Map<String, IndexStorePlugin.SnapshotCommitSupplier> snapshotCommitSuppliers = pluginsService.filterPlugins(
                IndexStorePlugin.class
            )
                .stream()
                .map(IndexStorePlugin::getSnapshotCommitSuppliers)
                .flatMap(m -> m.entrySet().stream())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
            if (DiscoveryNode.isMasterNode(settings)) {
                clusterService.addListener(new SystemIndexManager(systemIndices, client));
            }
            final RerouteService rerouteService = new BatchedRerouteService(clusterService, clusterModule.getAllocationService()::reroute);
            rerouteServiceReference.set(rerouteService);
            clusterService.setRerouteService(rerouteService);
            final IndicesService indicesService = new IndicesService(
                settings,
                pluginsService,
                nodeEnvironment,
                xContentRegistry,
                analysisModule.getAnalysisRegistry(),
                clusterModule.getIndexNameExpressionResolver(),
                indicesModule.getMapperRegistry(),
                namedWriteableRegistry,
                threadPool,
                settingsModule.getIndexScopedSettings(),
                circuitBreakerService,
                bigArrays,
                scriptService,
                clusterService,