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/main/java/org/elasticsearch/datastreams/DataStreamsPlugin.java
public static final Setting<TimeValue> TIME_SERIES_POLL_INTERVAL = Setting.timeSetting(
        "time_series.poll_interval",
        TimeValue.timeValueMinutes(5),
        TimeValue.timeValueMinutes(1),
        TimeValue.timeValueMinutes(10),
        Setting.Property.NodeScope,
        Setting.Property.Dynamic
    );
    public static final Setting<TimeValue> LOOK_AHEAD_TIME = Setting.timeSetting(
        "index.look_ahead_time",
        TimeValue.timeValueHours(2),
        TimeValue.timeValueMinutes(1),
        TimeValue.timeValueDays(7),
        Setting.Property.IndexScope,
        Setting.Property.Dynamic
    );
    public static final String LIFECYCLE_CUSTOM_INDEX_METADATA_KEY = "data_stream_lifecycle";
    
    private final SetOnce<UpdateTimeSeriesRangeService> updateTimeSeriesRangeService = new SetOnce<>();
    private final SetOnce<DataStreamLifecycleErrorStore> errorStoreInitialisationService = new SetOnce<>();
    private final SetOnce<DataStreamLifecycleService> dataLifecycleInitialisationService = new SetOnce<>();
    private final Settings settings;
    public DataStreamsPlugin(Settings settings) {
        this.settings = settings;
    }
    protected Clock getClock() {
        return Clock.systemUTC();