When BulkProcessor retries failed item requests then the same `IndexRequest` instance are used in a new `BulkRequest` for a retry. This is a problem when `BulkProcessor` is used from within Elasticsearch (this is the case for deprecation index logging, ilm/slm history and more), because the bulk request transport action may have set the `autoGeneratedTimestamp` field. 

If that is the case then error such a these may appear in the logs and the `BulkProcessor` retry fails:

```
Bulk write of 1 deprecation logs failed: autoGeneratedTimestamp should not be set externally
java.lang.IllegalArgumentException: autoGeneratedTimestamp should not be set externally
	at org.elasticsearch.action.bulk.TransportBulkAction.doInternalExecute(TransportBulkAction.java:223) ~[elasticsearch-7.16.1.jar:7.16.1]
	at org.elasticsearch.action.bulk.TransportBulkAction$1.doRun(TransportBulkAction.java:192) ~[elasticsearch-7.16.1.jar:7.16.1]
	at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26) [elasticsearch-7.16.1.jar:7.16.1]
	at org.elasticsearch.action.bulk.TransportBulkAction.doExecute(TransportBulkAction.java:200) [elasticsearch-7.16.1.jar:7.16.1]
	at org.elasticsearch.action.bulk.TransportBulkAction.doExecute(TransportBulkAction.java:91) [elasticsearch-7.16.1.jar:7.16.1]
```

In the bulk transport action, for each bulk item request of the `BulkRequest` its `process()` method is invoked. Incase of `IndexRequest` when no _id has been specified an id is generated and in that case the `IndexRequest#autoGeneratedTimestamp ` field is set. If the bulk request fails at a later stage, for example because there was no capacity on the node that hosts a primary shard to index documents then the bulk response will contain a `EsRejectedExecutionException` exception in the `BulkResponse` for the bulk items this failure occurred for. This is normal behaviour. However the `BulkProcessor` has retry logic and `EsRejectedExecutionException` is deemed retry-able. The `BulkProcessor` [reuses the same bulk request item instances ](https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/action/bulk/Retry.java#L139)(`IndexRequest` in this case) upon retry. This causes the failure mentioned in the stack trace.
server/src/main/java/org/elasticsearch/action/bulk/BulkRequest.java
implements
        CompositeIndicesRequest,
        WriteRequest<BulkRequest>,
        Accountable,
        RawIndexingDataTransportRequest {
    private static final long SHALLOW_SIZE = RamUsageEstimator.shallowSizeOfInstance(BulkRequest.class);
    private static final int REQUEST_OVERHEAD = 50;
    /**
     * Requests that are part of this request. It is only possible to add things that are both {@link ActionRequest}s and
     * {@link WriteRequest}s to this but java doesn't support syntax to declare that everything in the array has both types so we declare
     * the one with the least casts.
     */
    final List<DocWriteRequest<?>> requests = new ArrayList<>();
    private final Set<String> indices = new HashSet<>();
    protected TimeValue timeout = BulkShardRequest.DEFAULT_TIMEOUT;
    private ActiveShardCount waitForActiveShards = ActiveShardCount.DEFAULT;
    private RefreshPolicy refreshPolicy = RefreshPolicy.NONE;
    private String globalPipeline;
    private String globalRouting;
    private String globalIndex;
    private Boolean globalRequireAlias;
    private long sizeInBytes = 0;
    public BulkRequest() {}
    public BulkRequest(StreamInput in) throws IOException {
        super(in);
        waitForActiveShards = ActiveShardCount.readFrom(in);