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/test/java/org/elasticsearch/datastreams/LookAHeadTimeTests.java
var indexSettings = Settings.builder().put(DataStreamsPlugin.LOOK_AHEAD_TIME.getKey(), "1m").build();
            var e = expectThrows(IllegalArgumentException.class, () -> updateIndexSettings(indexSettings));
            assertThat(
                e.getMessage(),
                equalTo(
                    "failed to parse value [1m] for setting [index.look_ahead_time], must be lower than setting "
                        + "[time_series.poll_interval] which is [3m]"
                )
            );
        }
    }
    public void testLookAheadTimeSettingHigherThanTimeSeriesPollIntervalSetting() throws IOException {
        var clusterSettings = Settings.builder().put(DataStreamsPlugin.TIME_SERIES_POLL_INTERVAL.getKey(), "10m").build();
        updateClusterSettings(clusterSettings);
        var indexSettings = Settings.builder().put(DataStreamsPlugin.LOOK_AHEAD_TIME.getKey(), "100m").build();
        updateIndexSettings(indexSettings);
    }
    private void updateClusterSettings(Settings settings) {
        clusterAdmin().updateSettings(new ClusterUpdateSettingsRequest().persistentSettings(settings)).actionGet();
    }
    private void updateIndexSettings(Settings settings) throws IOException {
        try {
            createIndex("test");
        } catch (ResourceAlreadyExistsException e) {
            
        }
        indicesAdmin().updateSettings(new UpdateSettingsRequest(settings)).actionGet();
    }
}