Provide per-pipeline timestamps

[peterpramb]

Describe the feature:

Currently _ingest.timestamp seems to be only set initially when entering an ingest pipeline at the first time. However to measure per-pipeline ingest delays (as discussed in elastic/ecs#453) an updated timestamp per pipeline would be needed.

Of course one can retrieve the date via script, but it would be more convenient to directly access it via metadata, either by letting _ingest.timestamp be updated on each pipeline call or by introducing a new ingest metadata field.

Here is a simple test case, Elasticsearch version is 6.7.1:

PUT /_ingest/pipeline/testing2-ingest-pipeline
{
    "processors": [
        {
            "append": {
                "field": "ingest.timestamps",
                "value": "{{_ingest.timestamp}}"
            }
        },
        {
            "pipeline": {
                "name": "testing2-chained-pipeline"
            }
        }
    ]
}

PUT /_ingest/pipeline/testing2-chained-pipeline
{
    "processors": [
        {
            "append": {
                "field": "ingest.timestamps",
                "value": "{{_ingest.timestamp}}"
            }
        }
    ]
}

POST /testing2-chained/_doc/?pipeline=testing2-ingest-pipeline
{
  "field": "value"
}

GET /testing2-chained/_doc/S25crGoBkBB05GCNSA5t
{
    "_id": "S25crGoBkBB05GCNSA5t",
    "_index": "testing2-chained",
    "_primary_term": 1,
    "_seq_no": 0,
    "_source": {
        "field": "value",
        "ingest": {
            "timestamps": [
                "2019-05-12T14:04:50.054Z",
                "2019-05-12T14:04:50.054Z"
            ]
        }
    },
    "_type": "_doc",
    "_version": 1,
    "found": true
}

[peterpramb]

Is it possible that _ingest.timestamp is already properly updated and milisecond resolution is simply not enough here?

[elasticmachine]

Pinging @elastic/es-core-features

[peterpramb]

Probably not a good idea, _ingest.timestamp should stay as is. One can get the current time via script if really needed.