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
private void writeBody(StreamOutput out) throws IOException {
        if (out.getVersion().before(Version.V_8_0_0)) {
            out.writeOptionalString(MapperService.SINGLE_MAPPING_NAME);
        }
        out.writeOptionalString(id);
        out.writeOptionalString(routing);
        out.writeBytesReference(source);
        out.writeByte(opType.getId());
        out.writeLong(version);
        out.writeByte(versionType.getValue());
        out.writeOptionalString(pipeline);
        if (out.getVersion().onOrAfter(Version.V_7_5_0)) {
            out.writeOptionalString(finalPipeline);
        }
        if (out.getVersion().onOrAfter(Version.V_7_5_0)) {
            out.writeBoolean(isPipelineResolved);
        }
        out.writeBoolean(isRetry);
        out.writeLong(autoGeneratedTimestamp);
        if (contentType != null) {
            out.writeBoolean(true);
            XContentHelper.writeTo(out, contentType);
        } else {
            out.writeBoolean(false);
        }
        out.writeZLong(ifSeqNo);
        out.writeVLong(ifPrimaryTerm);
        if (out.getVersion().onOrAfter(Version.V_7_10_0)) {
            out.writeBoolean(requireAlias);
        }