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
additionalLookAheadTimeValidation(value, timeSeriesPollInterval);
        });
        List<RestHandler> handlers = new ArrayList<>();
        handlers.add(new RestCreateDataStreamAction());
        handlers.add(new RestDeleteDataStreamAction());
        handlers.add(new RestGetDataStreamsAction());
        handlers.add(new RestDataStreamsStatsAction());
        handlers.add(new RestMigrateToDataStreamAction());
        handlers.add(new RestPromoteDataStreamAction());
        handlers.add(new RestModifyDataStreamsAction());
        handlers.add(new RestPutDataStreamLifecycleAction());
        handlers.add(new RestGetDataStreamLifecycleAction());
        handlers.add(new RestDeleteDataStreamLifecycleAction());
        handlers.add(new RestExplainDataStreamLifecycleAction());
        return handlers;
    }
    @Override
    public Collection<IndexSettingProvider> getAdditionalIndexSettingProviders(IndexSettingProvider.Parameters parameters) {
        return List.of(new DataStreamIndexSettingsProvider(parameters.mapperServiceFactory()));
    }
    @Override
    public void close() throws IOException {
        try {
            IOUtils.close(dataLifecycleInitialisationService.get());
        } catch (IOException e) {
            throw new ElasticsearchException("unable to close the data stream lifecycle service", e);
        }
    }
}