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/DataStreamIndexSettingsProviderTests.java
now,
                settings,
                null
            )
        );
        assertThat(
            e.getMessage(),
            equalTo(
                Strings.format(
                    "backing index [%s] in tsdb mode doesn't have the [index.time_series.end_time] index setting",
                    DataStream.getDefaultBackingIndexName(dataStreamName, 1, twoHoursAgo.toEpochMilli())
                )
            )
        );
    }
    public void testGetAdditionalIndexSettingsNonTsdbTemplate() {
        Metadata metadata = Metadata.EMPTY_METADATA;
        String dataStreamName = "logs-app1";
        Settings settings = Settings.EMPTY;
        Settings result = provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            false,
            metadata,
            Instant.ofEpochMilli(1L),
            settings,
            null
        );
        assertThat(result.size(), equalTo(0));
    }