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
/*
         * Set the auto generated timestamp so the append only optimization
         * can quickly test if this request *must* be unique without reaching
         * into the Lucene index. We lock it >0 because UNSET_AUTO_GENERATED_TIMESTAMP
         * has a special meaning and is a negative value. This optimiation will
         * never work before 1970, but that's ok. It's after 1970.
         */
        autoGeneratedTimestamp = Math.max(0, System.currentTimeMillis());
        String uid = UUIDs.base64UUID();
        id(uid);
    }
    public void checkAutoIdWithOpTypeCreateSupportedByVersion(Version version) {
        if (id == null && opType == OpType.CREATE && version.before(Version.V_7_5_0)) {
            throw new IllegalArgumentException(
                "optype create not supported for indexing requests without explicit id until all nodes " + "are on version 7.5.0 or higher"
            );
        }
    }
    @Override
    public void writeTo(StreamOutput out) throws IOException {
        checkAutoIdWithOpTypeCreateSupportedByVersion(out.getVersion());
        super.writeTo(out);
        writeBody(out);
    }
    @Override
    public void writeThin(StreamOutput out) throws IOException {
        checkAutoIdWithOpTypeCreateSupportedByVersion(out.getVersion());
        super.writeThin(out);
        writeBody(out);
    }