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
}
    private static Map<?, ?> getIndex(String indexName) throws IOException {
        var getIndexRequest = new Request("GET", "/" + indexName + "?human");
        var response = client().performRequest(getIndexRequest);
        assertOK(response);
        return entityAsMap(response);
    }
    private static Instant getStartTime(Map<?, ?> getIndexResponse) throws IOException {
        assert getIndexResponse.keySet().size() == 1;
        String topLevelKey = (String) getIndexResponse.keySet().iterator().next();
        String val = ObjectPath.evaluate(getIndexResponse.get(topLevelKey), "settings.index.time_series.start_time");
        return Instant.from(DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).parse(val));
    }
    private static Instant getEndTime(Map<?, ?> getIndexResponse) throws IOException {
        assert getIndexResponse.keySet().size() == 1;
        String topLevelKey = (String) getIndexResponse.keySet().iterator().next();
        String val = ObjectPath.evaluate(getIndexResponse.get(topLevelKey), "settings.index.time_series.end_time");
        return Instant.from(DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).parse(val));
    }
    static String formatInstant(Instant instant) {
        return DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).format(instant);
    }
    static String formatInstantNanos(Instant instant) {
        return DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME_NANOS.getName()).format(instant);
    }
    static Instant parseInstant(String input) {
        return Instant.from(DateFormatter.forPattern(FormatNames.STRICT_DATE_OPTIONAL_TIME.getName()).parse(input));
    }
}