### Description

The `sendRequest` method is asynchronous and notifies a 
`TransportResponseHandler` when it completes the excecution
(successfully or unsuccessfully). 

When opening the connection we only catch `NodeNotConnectedException`
( https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/transport/TransportService.java#L719 ) which is currently fine as no implementations
throw other type of exception however, it'd be best to catch `Exception` here
so the `handler` gets notified of potential exceptions (as opposed to the caller
having to `try/catch` the call to `sendRequest`)
server/src/main/java/org/elasticsearch/cluster/coordination/CoordinationDiagnosticsService.java
import org.elasticsearch.action.admin.cluster.coordination.CoordinationDiagnosticsAction;
import org.elasticsearch.cluster.ClusterChangedEvent;
import org.elasticsearch.cluster.ClusterStateListener;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.service.ClusterApplierService;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.common.io.stream.Writeable;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.util.concurrent.EsRejectedExecutionException;
import org.elasticsearch.common.util.concurrent.ThreadContext;
import org.elasticsearch.core.Nullable;
import org.elasticsearch.core.Releasable;
import org.elasticsearch.core.Releasables;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.threadpool.Scheduler;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.transport.ConnectionProfile;
import org.elasticsearch.transport.TransportRequestOptions;
import org.elasticsearch.transport.TransportService;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Locale;
import java.util.Map;