### 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
* repeats doing that until cancel() is called on all of the Cancellable that this method inserts into cancellables. This method
     * exists (rather than being just part of the beginPollingClusterFormationInfo() above) in order to facilitate unit testing.
     * @param nodeResponseConsumer A consumer for any results produced for a node by this method
     * @param cancellables The Map of Cancellables, one for each node being polled
     */
    
    void beginPollingClusterFormationInfo(
        Collection<DiscoveryNode> masterEligibleNodes,
        BiConsumer<DiscoveryNode, ClusterFormationStateOrException> nodeResponseConsumer,
        Map<DiscoveryNode, Scheduler.Cancellable> cancellables
    ) {
        masterEligibleNodes.forEach(masterEligibleNode -> {
            Consumer<ClusterFormationStateOrException> responseConsumer = result -> nodeResponseConsumer.accept(masterEligibleNode, result);
            try {
                cancellables.put(
                    masterEligibleNode,
                    fetchClusterFormationInfo(
                        masterEligibleNode,
                        responseConsumer.andThen(
                            rescheduleClusterFormationFetchConsumer(masterEligibleNode, responseConsumer, cancellables)
                        )
                    )
                );
            } catch (EsRejectedExecutionException e) {
                if (e.isExecutorShutdown()) {
                    logger.trace("Not rescheduling request for cluster coordination info because this node is being shutdown", e);
                } else {
                    throw e;
                }
            }