Currently the `index.look_ahead_time` index setting controls the `index.time_series_end_time` index setting when rolling over a tsdb data stream. The `index.look_ahead_time` setting default to 2 hours, which means the by default the `index.time_series_end_time` index setting of the new backing index will be 2 hours later than the `index.time_series.start_time` index setting. The the `index.time_series.start_time` and `index.time_series_end_time` index settings  control what `@timestamp` are allowed to be indexed.

Additionally the `index.look_ahead_time` setting is used to generate the `index.time_series.start_time` and `index.time_series_end_time` index settings when creating a new data stream for the first backing index. Based on the defaults of `index.look_ahead_time` setting, the  `index.time_series.start_time` index is set to current time - 2 hours and the `index.time_series_end_time` index setting is set to current time + 2 hours.

However for indexing the initial data setting the `index.time_series.start_time` index setting to 2 hours in the past sometimes creates too small time window. For example because data gets indexed starting from midnight that day. 

The idea is to add a `index.look_back_time` setting that controls how to generate the `index.time_series.start_time` index setting when creating the data stream (for the first backing index). This allows for creating a first backing index that accepts data that is earlier then current time - $index.look_ahead_time, without affecting how `index.time_series.start_time` and `index.time_series_end_time` index settings are generated during rollover.

I think a good default for the `index.look_back_time` setting would be 24 hours. Meaning that by default the `index.time_series.start_time` index setting for the first backing index of a new data stream would be current time - 24 hours.

Relates to elastic/integrations#7345
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
"name": "dog",
                    "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9",
                    "ip": "10.10.55.3",
                    "network": {
                        "tx": 1434595272,
                        "rx": 530605511
                    }
                }
            }
        }
        """;
    private static final String BULK =
        """
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507", "ip": "10.10.55.1", "network": {"tx": 2001818691, "rx": 802133794}}}}
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "hamster", "uid":"947e4ced-1786-4e53-9e0c-5c447e959508", "ip": "10.10.55.1", "network": {"tx": 2005177954, "rx": 801479970}}}}
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "cow", "uid":"947e4ced-1786-4e53-9e0c-5c447e959509", "ip": "10.10.55.1", "network": {"tx": 2006223737, "rx": 802337279}}}}
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "rat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959510", "ip": "10.10.55.2", "network": {"tx": 2012916202, "rx": 803685721}}}}
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9", "ip": "10.10.55.3", "network": {"tx": 1434521831, "rx": 530575198}}}}
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "tiger", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea10", "ip": "10.10.55.3", "network": {"tx": 1434577921, "rx": 530600088}}}}
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "lion", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876e11", "ip": "10.10.55.3", "network": {"tx": 1434587694, "rx": 530604797}}}}
            {"create": {}}
            {"@timestamp": "$now", "metricset": "pod", "k8s": {"pod": {"name": "elephant", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876eb4", "ip": "10.10.55.3", "network": {"tx": 1434595272, "rx": 530605511}}}}
            """;