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/index/IndexRequest.java
* @see org.elasticsearch.client.internal.Requests#indexRequest(String)
 * @see org.elasticsearch.client.internal.Client#index(IndexRequest)
 */
public class IndexRequest extends ReplicatedWriteRequest<IndexRequest> implements DocWriteRequest<IndexRequest>, CompositeIndicesRequest {
    private static final long SHALLOW_SIZE = RamUsageEstimator.shallowSizeOfInstance(IndexRequest.class);
    /**
     * Max length of the source document to include into string()
     *
     * @see ReplicationRequest#createTask
     */
    static final int MAX_SOURCE_LENGTH_IN_TOSTRING = 2048;
    private static final ShardId NO_SHARD_ID = null;
    private String id;
    @Nullable
    private String routing;
    private BytesReference source;
    private OpType opType = OpType.INDEX;
    private long version = Versions.MATCH_ANY;
    private VersionType versionType = VersionType.INTERNAL;
    private XContentType contentType;
    private String pipeline;
    private String finalPipeline;
    private boolean isPipelineResolved;
    private boolean requireAlias;
    /**
     * Value for {@link #getAutoGeneratedTimestamp()} if the document has an external
     * provided ID.
     */
    public static final long UNSET_AUTO_GENERATED_TIMESTAMP = -1L;
    private long autoGeneratedTimestamp = UNSET_AUTO_GENERATED_TIMESTAMP;